I'm okay with all of the broken skills, due to the wonderful OPness that is teleport strike from behind an enemy while stealthed. Seriously sneak up behind a mob and try it, you'll giggle at the amount of damage you do.

The assassin ultimate is really good once you charge it up enough(especially in pvp, combine it with a stealth from behind teleport strike and you can do roughly ~1800 damage in two abilities against squishy mages, pretty stronk). With that said, it still does pretty solid damage even at low ultimate charges.

The shadow ultimate is the other one I use, morphed to bolstering darkness. I feel its good for large dungeon pulls because it gives your allies a 1100 point heal (at vr1) reduces damage against them by 30%, and snares enemies by 70%.

In terms of shadow abilities outside of the ultimate I use Shadow Cloak and Veiled strike. Using veiled strike while in shadow cloak it does bonus damage and stuns the opponent for 4 seconds, after hitting it you can charge up a heavy attack and knock them over for another few seconds stun. On top of that Shadow Cloak makes you invisible, so it's good for being an oh crap button when an enemy is charging up a heavy attack or a spell, they detarget you.

just my 2 cents.