Celebi, Mew and Lati@s are the only ones that don't land you instantly on my hate list. Not sure why, I guess because they don't seem any stronger than the other pokemon in their tier.
Deoxys, Genesect, Heatran, Suicune, Lake Trio and Darkrai all make me wish I could track you down and mail you a box full of anthrax and horse porn. Fuck all of those and fuck everything they do, most obnoxious little shits in their respective tiers, and at least three of them need to be banned to the tier above where they are.
If you use legendaries in-game you should consider suicide. The only exceptions are the recent box legendaries and ONLY to use them for the story battle that always immediately follows their capture.