For PvE, it depends on the encounter and your perk loadout. For instance, if you rock Gift of the Sun with fusion grenades, [u][b]Starfire Protocol[/b][/u]'s exotic perk is useless. You should use ingnite (or angel of light) instead. Starfire protocol requires that you use fusion grenades for it to be worth the exotic slot. [b][u]Heart of the Praxic Fire[/u][/b] is almost always a solid choice, but it's usefulness is lost if you use fireborn and only pop radiance when self-rezzing. The extra cool downs are best used offensively to make orbs for teammates. Grenades should be chosen by what you are personally most effective with. Some people swear by forebolt grenades with ignite and viking funeral (especially for PvP), but I personally feel like I put out more damage with fusion. Solar grenades can be useful in certain situations (see below). I haven't used the [b][u]Ram[/u][/b] yet, but my understanding is that it is better for voidwalkers, although it has benefits for sunsinger as well. Solar grenades are often useful for controlling enemy movement, especially in hive waves. [b][u]Sunbreakers[/u][/b] increase the burn time for solar grenades. The extra burn time for solar grenades is nice when using them for that, but I don't think the extra burn time makes up for the extra cool down yoy can get with HotPF. [b][u]Claws of ahamakara[/u][/b] can be useful for having a second charge of flame shield. TBH, though, I have never really felt viable with that extra melee charge. Maybe now by using a weapon with army of one, it might be worth having the extra charge and running gift of the Sun so you can have the extra charges to store the energy regained through army of one. [b][u]Apotheosis veil[/u][/b] has a very niche use. If you use radiance defensively, the heal can be nice, but I feel like exotic perks should enhance damage output in addition to adding to survivability. AV pretty much is just defensive. Finally, [b][u]light beyond nemesis[/u][/b] is extremely useful in PoE. In the higher level encounters, your teammates are going to need orbs. LBN helps increase the output. It also helps revive and get revived faster. Unless your team is super MLG pro, chances are someone on your team is going to need a revive at some point. I hope this was somewhat helpful. Edit: for crucible, [b][u]void fang vestments[/u][/b] are super cheesy, but useful. You always spawn with grenade energy, so fusion grenades + gift of sun + void fang means two fusion grenade kills every time you spawn. Also, I didn't mention the obvious voidwalker helm Obsidian mind. No for sunsinger. Inverse shadow comes on legendary helms, so don't use OM as a sunsinger.

If you're mainly doing PvE then you're best bet is Heart of the Praxic Fire, easily. It just an all around good exotic.

For PvE, Heart of the Praxic Fire, with Starfire Protocol taking a close second. PvP, either the two above, or the Sunbreakers IF you like Solar grenades for Crucible.

PRAISE THE SUN

The ram with radiant skin and flame shield

It's all on playstyle. I play support Sunsinger (orbs n' shit), so I find Praxic Fire helps a lot. If you want to go for more of a tank build, claws or ram are for you.

For PvP, the Ram with Radiant skin makes you the ultimate TANK. The boosted armor from the ram plus the radiant skin overshield allow you to eat a full clip of thorn and keep going. I have yet to test this idea, but I'm pretty sure you can tank a GG shot as well.

I use voidfang vestiments;) it's the best cheese in the game

You'll get a lot of differing opinions, but Claws of Ahamkara and Flame Shield has always been a solid choice for me.