Boss damage: raze lighter
Ranged option: bolt caster
Enemy clearing: dark drinker
Normal sword enemy clearing/parkour
Infinite sword: the same as the normal one just looks different
Raze Lighter in PvP Bolt Caster in PvP legendary swords if you don't want to "waste an exotic slot" on a sword :3 (year one vanilla players will understand)
They are all good, and each has different uses.
Bolt caster is best for pvp. Also, it's ok for pve if you need some additional range options.
Raze has the highest DPS. It will dump it's ammo the fastest.
Dark drinker is probably my favorite. It's the most versatile.
AoE for trash is awesome, especially if you couple it with defender or nightstalker and the exotic helm that give orbs for heavy kills.
Also, it does more damage per hit than raze. Raze's animation is much faster though, which is why raze has a higher DPS, just lower damage per hit and lower damage potential for total ammo use.
How I use the swords
Raze lighter:
Use on warlock sun singer
Run at boss
Unload(?) ammo
Die
Self resurrect
Run
Bolt caster:
Use on hunter w/ shade step
Endless pvp fun as you electrocute your opposition
Dark drinker:
Armor of light Titan bubble, you get the picture
Who cares about legendary swords
Raze Lighter for Single Target. Does Massive Damage
Dark Drinker for AoE. Wipes out most adds with a single activation around you, in about the same distance as FoH
Bolt Caster for Range. Not too sure on damage on this one, seems to do great in PvP if you're willing to use an exotic slot for it.
In all honesty, I like Raze Lighter because I generally use Heavy for bosses anyways. Bosses or shielded targets anyway.