The Hunter exotic on sale this weekend is worth picking up. There are many situations where going invisible can be far more useful than a 4th GG shot - getting a tricky revive, hiding from enemies to recover health, picking up some faraway ammo during an elemental burn activity, capping a mine in Prison of Elders... don't pass it up.
As for weapons, Ice Breaker is a classic. Before Gjallarhorn's DPS potential was figured out, Ice Breaker was the poster boy everyone deemed the most important element in a PvE arsenal.
Prison of Elders has a lot of activities with Juggled and Majors in your face. Invective is a great asset - a shotgun with regenerating ammo works wonders in PoE. In PvE, my exotic slot belongs to Gjallarhorn, Ice Breaker or Invective most of the time unless I'm just messing around and want to play with silly builds.
If you play PvP, that's a whole other world though. Stuff you'd never look twice at in PvE become your most powerful tools in the Crucible. Knucklehead Radar for radar while aiming your primary is a favorite of mine for PvP.