that deathclaw that sneaks up behind you. Nothing makes you jump like a surprise Deathclaw from behind with your weapon holstered
Runner up is the flower enemies in super paper mario that put you to sleep, especially in world 3-3 (That is the tree climbing one right?)
sorry if already said
Edit: Enclave troops and Talon Company (mostly when you have a good karma level), enough said
then anything that runs in any game made by blizzard, especially when its in a cramped dungeon in WoW (deadmines)
Edit 2: oh shoot i just thought of more.... plenty more... *goes for list*
we got, Gomba's (when they kill you), Those plant... things in zelda that require you to shoot the seeds back into them... koopa shells, Snipers in killzone, end game bosses in final fantasy games, ummm...... Redead in zelda, flying enemies in anything... and let us not forget, your fellow players in online VS. matches or in some cases... your allies in online games (L4D friendly fire)