Okay, so this is pretty criminal that this hasn't been mentioned yet..
Armed and Dangerous: The Upsy Daisy (I think thats what it's called at least)
Imagine two punch-knife style handles, with a corkscrew instead of the blade. You stick them in the ground.
The world then turns upside down. You hang from the floor while the enemies FALL INTO THE SKY, the world turns back to normal, and SPLAT.
Yes. This weapon turns the world upside down, but keeps gravity in the same direction.
Also from A&D: The Land Shark Launcher.
You fire a small grey baby shark that has been modified to swim and breathe in earth. The Jaws Music plays and a few seconds later a rapdily-matured SHARK bursts out of SOLID ROCK and swallows enemies whole. HELL YES.
Bioshock had some fun ones, especially the hand that shoots bees, but really the only plasmid I used reliably (when not forced to by Lot192) was the shocker one.