Once you unlock Quiet Casting (Illusion) your Shouts are silent too. Become Ethereal is nice for getting the drop on people from instadeath heights. Also, Wuld (Whirlwind Sprint, I generally just use the first Word unless there's a long Gap) is handy to get close with a quickness. Ice Form is Paralysis anyone can use with a 100% chance of success, and you can hit multiple targets if they're close enough together. Ironically enough (Because of where you find it) I'm not a big fan of Marked for Death, anyone hit by it all of a sudden is alerted, and 9 times out of 10, you're better off sticking a dagger, or two in their kidneys. Dragonrend is nice to bring down Dragons which you may be weak against because of character focus. Elemental Fury really helps with clearing out a crouded room, especially stacked with Dual Flurry daggers, or a Sword/Dagger combo like the Blade of Woe, and Windshear. (The BoW in the offhand, Windshear isn't technically enchanted, so you get the Dagger accelleration for it being in your offhand, and the EF accelleration from the scimitar in your right, plus the Stagger effect. This is why I generally just Temper Windshear, but leave it unenchanted.)
Also Aura Whisper is truly great so you know where everyone is, and there's no surprises (for you!) Probably the best shout for an Assassin, and it's already silent. The distraction one (I can't recall the name right off hand) isn't worth the fights up on Shearpoint just to get it. You can shoot an arrow, or cast a projectile spell (as long as it's quiet, and not too bright, I generally use Calm, or Raise Zombie) for the exact same effect.