Best: Praedyth's Revenge. From the VoG, void damage sniper rifle with the most insane reloads I've ever seen (added to further thanks to other similar buffs my character has). Hit the sweet spot and it explodes targets causing area of effect damage explosions ( e.g. vapourises adjacent Vandals when you snipe the guy next to him). Was taking ~8000 a pop off an animated Atheon, with 6 quick, steady shots from the magazine and a total of 24 it's dangerous to be around.
Oldy but goody: Bronzed Miyamoto-D. It's only a rare and I've had basically the Legendary equivalent and recycled but kept this one. It got me through much of the earlier parts of the game and was a ton of fun to use. Insane RoF with miniscule impact, but had the great effect of helping put some strike bosses (such as the psions) into a kind of shocked/stunned state when also fired upon by others in the fireteam. Similar with the Public Events to destroy the target, kept them from running away too fast. This one is in the bank, never letting go, in fact I might pop it back into my inventory just for fun when I start my new Space Jesus tomorrow, will give me something to aim for before level 20.
Cosmetic: Fragment of the Prime. It's sparkly, nothing more than that. Other Side of Nothing has nice purple mist, Forgotten Force has nice fire, but I prefer the cosmic bling of this one.