Altaria (It had Ice Beam, meaning that it nuked most of the Elite 4. And by that, I mean about 3/4 of their pokemon), swiftly followed by Blaziken (who pretty much had the first (darkness) and third (Ice) of the Elite 4 in Pokemon Ruby to himself).
Well I started playing about 2 years ago from a couple year hiatus (it was uncool in middle school. It still is uncool in high school, but I don't give a shit what other people think, I think it's cool) and have played numerous different games. I started with Emerald (which I didn't enjoy that much) then moved to FireRed then up to Platinum and now to SoulSilver. I have 15 level 100s. My first was Rayquaza. I got my Blaziken there afterwards and have had numerous ones since. Tyranitar is one of my personal favorites, along with Blissey.
Dragonite! i loved the big ugly bastard despite that fact that he was a fat orange thing where his predecessors had been sleek, beautiful, elegant creatures. In fact after i caught dratini i never really used much else apart from occasionally my Charizard or my Jynx.