I'm only talking about powers.
Smashguard's smash boosted by charge and melee damage
Build
(smashdept's damage comes close but he doesn't get the charge boost like the vanguard does)
Awakened collector's dark sphere
Build
Drell assasin's recon mine
Build
Shadow strike
Build
Not counting grenades, these four are the highest I can think of.
When it comes to weapons, a DI javelin can mathematically do more damage than GI javelin, but it requires very specific circumstances. Because of that, the DI javelin has the highest spike damage.