Getting Old Marta to cut your intestines up in Planescape: Torment.
"Scroll of Icarian Flight" + mountain = funnest thing to do in Morrowind.
Waiting 10 minutes underwater in the Secret of Monkey Island.
Fighting the Megadragon in Might & Magic 5. My parties were cheesed up enough to beat him (except for my very first one, but give me a break on that) in a few rounds. Except once, his breath weapon (which hits the entire party), which also has a chance of instant-killing... Well, 6 eradicated party members and still a cast of teleport away from engaging. Yummy.
One of my favourite kills is from that game too; the jump / mass distortion combo to break Barkman.
Old NES game, Crystalis. One of the bosses could turn you into an absolutely harmless ooze. Hope you have a Fruit of Repun.
Back in the glory days of EQ, gathering an entire newbie zone (The Warrens was a particular favourite) and dropping the PBAoE's. Four minute computer locks ftw.
Not really a 'kill' per se, but way back on the PS1, the good old Jet Moto games. Grapple pole, need to do a 180 right on the spot. Best feeling ever is when you're fighting for first place, you come to this section, and you introduce the bottom of your bike to the back of their head. On the Hotshot map in JM2, that happened all the time, and it was so awesome.
Fully unlocked, fully charged, 'O' in Heavenly Sword. A well timed triangle was awesome, too. Or that part where walking into an enemy kills it, and just running in circles.