Kyogre is probably one of the best ways to cheat ever created. Use Calm-Mind a couple of times, and you'll have a one-hit KO'ing machine, with a Sp. Defense so high that almost nothing can take it down.
I never acutally use legendaries (because what's the challenge in that, right? xD), but if I had to pick a favorite, I'd probably say Rayquaza. It looks cool. =P