Grenades are difficult to state which one is "best", and in the end it will be personal opinion as to what one you take, but i can explain in a few areas what grenade is best for what.
This grenade is currently one of my favourites, and the one i have used the most. It has the potential to kill the most people, but also the ability to kill absolutely no one.
The damage it does can outright kill another player... 100% shielded to 0% health. The player has to stand around the grenade a lot, but in crucible you can very easily get 2-3 kills with this grenade nicely landed inside a zone. This makes it incredibly useful in control as a way to zone out of the capture point, or even hallways / corridors. ( As a side note, the damage it does when someone runs through it has been enough to save me near death from a 2nd player charging in for the kill )
However this grenade is very easily countered , a player will see your little mini-sun and walk away from it...or fight around around it, it is very hard to use this grenade if you know the enemy doesn't need to stand in a confined space for like 5 seconds.
This is a strange grenade and i think you have to use a few times to understand its usefulness, as it is the only grenade in our arsenal that doesn't outright kill an enemy . It will take a player from 100% to about 30%-ish (health range). However it has one key feature homing projectiles that will track around a corner and sink that kill you wanted.
Another key point of this grenade is the fact it is direct damage and not a DoT... predicting an enemy to be running in to salvage a probe? Launch a grenade into the area and then start running in to finish the kill, the enemy won't know what hit them (well they will in the killscreen..but you know what i'm saying).
This grenade suits aggressive play compared to defensive, and when in control i'll tend to stick with a solar grenade for its ability to zone out areas, i will happily admit that this grenade has been just as (if not more) useful in a few scenarios and i'm beginning to go over to this grenade more and more.
This is currently my least favourite grenade, and i have used it very little as its only use is a single kill (typically). It has one amazing ability that a stick with this grenade will kill your target and it has a little extra aiming to help get those single hit kills. Even if your grenade misses and is by the feet of your enemy will the kill occur but they have to be rather close for that to actually occur without a stick.
If you are a player who is a little spiteful and loves postmortem medals, then this is the grenade for you. When it is up, it will arguably always cause your life to at least be 1:1 if you land a stick on your killer... But that is about it.
When it comes to glide bonuses as dumb as it sounds, but it is basically what you prefer and what will suit your play style. I'd argue that focused control is the weakest of the three as it doesn't have a large effect. However between the other two it is simply personal preference.
The weakest of the three in my opinion, this gives extra directional control . Which i've never found to be an amazingly useful perk. But if this is what you are used to / like, there are no *real* benefits to choosing the other two.
The other end of the spectrum to control. This is focused on giving you a little burst of movement in the direction you are traveling... useful for closing a gap or running away. The initial burst of speed can be useful for getting around, but don't forget you have decreased accuracy in the air ( unless you have angel of light ) so try to use it as movement / utility and not a combat closer like blink.
The happy middle between both glides, it has a little of each other glide in it, for a smaller burst and a little better control . So if you are unsure on which one to take you can simply choose this one as a middle ground.
When it comes to your super, i believe all the choices are viable in situations but Song of Flame is clearly the weakest of the three. On another note, whilst you are super-ed kills with abilities / bullets do not count as a "kill with super" so you cannot complete that bounty as a sunsinger.
Whilst radiance is activated, scorch hits 1.7 times harder. So say you previously hit for 1000, you'd now hit for 1700. This is a beautiful added extra that is not explained anywhere but its incredibly useful.
Song of Flame
Do not pick this ability , if you are a lone wolf this perk provides absolutely nothing for you. Even if you are with a team of friends and you can be around them to boost their cooldowns... in most cases it won't be as beneficial to them as you would benefit from decreased damage or a surprise zombie mode.
My personal favourite and i believe it to be a must have, but i will argue the weaknesses and benefits fairly. Its tooltip doesn't give much information, but with a bit of testing you find out that it gives 50% damage reduction which means that SUROS being aimed at you will take twice as many bullets to take you down, and that will give you time to get the kill first.
This damage reduction will not stop you getting killed by any degree, you are still easily ripped through. However paired with the solar or fusion grenades to help weaken enemies as they stride towards you makes you a killing machine and the reduction on scorch just makes you a beast to take down.
Another viable choice of sunsinger, and makes you a zombie (kind of)... But i personally don't rate this ability and slowly people have adapted to it. Once you die, you can come back to life, and have radiance active which means that guy who was left 1-shot will be an easy kill and let you move on to kill others.
I have read others arguing that they will come back wth radiance on a zone, just to throw grenades and get a few kills before dying again and save the zone. Personally i find the idea that your super becomes incredibly situational and only lends itself to "revive and fight for 5 seconds" in those situations it isn't a useful skill and i would argue it is better to choose radiant skin, but if you believe this is a good choice please use it and give me some examples where it has been incredibly useful and change my opinion.
On that note Fireborn revives, still mean your death is counted. So any bounties ".... without dying" even if you use fireborn, you have still died.
This beautiful melee ability will save your life more than once, and it also has an easy perk choice. Although it would be nice to have other viable choices that aren't incredibly situational. Also as stated in radiance this ability does 1.7 times the damage whilst radiant .
The clear choice of melee perks , just hitting an enemy when this is active will cause yourself to get an extra shield. This will save your life so many times, and if you remember "oh my super is up" and melee the enemy first, you'll get your overshield and be able to sink your kill easier.
Just no, this is not a useful skill. The only time this will be useful is if you think you are going to die and try and push the enemy back but they will most likely still have line of sight and just shoot for the kill. Never chose this skill
Only useful when fighting multiple enemies against yourself and in those situations you are most likely going to die, this ability is rarely going to be beneficial to yourself and is more likely to kill you trying to set up for an awesome 2 for 1 brimstone kill than ever give you a 2 for 1.
Your choices here are between wisdom, spirit or force ... I honestly am under the opinion that recovery is the most important stat to gain in crucible, with agility second. So i would suggest wisdom, this will mean you recover your health/shield faster and can get around a map faster.
If i would say to not pick one, do not pick force... The armour benefit is so minimal and you've chosen agility over recovery. So you're missing out.
Increased duration of your super is an amazing ability, this will added another two seconds of radiance. Which will let you use another grenade or maybe 2 if you are really lucky.
Many people say this is an amazing thing to have, and i agree that it can benefit you a lot. However i tend to use radiant skin which means my super makes me stronger for specific times (like a point being capped really quickly by an enemy, so lots of enemies to grenade / take damage from) and this means i sit on my super ability and if you have chosen fireborn you will be sitting on your super a lot.
The best perk to choose, unless you have touch of flame... then its a close second.
The tooltip on this one is wrong, the tick does the same damage, but the burn lasts twice as long! So if you hit an enemy with scorch, they may tick for the same amount of damage but it'll be a longer burn and do 100% extra damage at the end, this also applies to burns applied through touch of flame.
Just no. Scorch will used mostly to gain a shield and not to sink the final blow in... Even then to lose out on 2 seconds of super for this weak ability would never be worth it. Just ignore this choice.
Another choice between three... Ancestral, Chaos or Divine . Again as stated before i rate recovery > agility > over, so my choice here is Ancestral Order . It will benefit your recovery the most, with added agility and some armour. But again pick which ever one you wish as it isn't a huge game changer.
Touch of Flame
If you don't have access to Gift of the Sun, always choose this perk , and if you have chosen this perk the viking funeral perk in column 6 becomes amazing. I have loved the DoT of solar damage on enemies... It stops them recovering any shield or health and a fair few times gets you a kill.
That makes this perk an amazing bonus and useful for a lot of lower level sunsingers, ( i ran sunsinger in crucible from level 2 ) so i have been in love with it for a long time, but the added grenade of Gift of the Sun is just too much extra damage and versatility to let go of, the extra grenade will always outdamage the burn.
Angel of Light
The day you want to be in the air and an easy target for longer... is the day you go 0.00, never pick this skill. It is no where near as useful as the other two.
People argue that in duels it can be used to get better line of sights, or suprise players... But in the end that is something that will happen maybe once a game... and not benefit you like the other skills.
Gift of The Sun
This has a catch, a hidden cooldown for the second grenade, not effected by strength CD reduction . Even with this surprise negative this skill is infallible compared to the others. Having two firebolt grenades at times means you can clear out control points with two grenades... and then keep moving, so you now have this ability to wipe out three people quickly (like a super) but you get two grenades every minute or so.
I have not found an argument / use for the other two over this skill. Always choose this.
If you follow my guide and have a similar play style to myself, i would always say Dead Orbit . In crucible as a sunsinger you want grenades and scorch up as much as possible and your super just reduces a cooldown on your abilities... so your super is "in essence" a lot of disc and str so how can you go wrong with dead orbit? You can't!
If you choose other factions, that's fine to. But i would say give the lack of a super / clear multiple enemies... your grenades are really important, so always get some disc and you can play defensive... or get some str and always play to have an overshield and a tanky as sunsinger.