Well, at the moment, I have it like this:
LeafGreen: Aerodactyl | Bulbasaur - I always end up getting Aerodactyl, because it can wup Lance's ass, and is generally awesome.
Emerald: Mightyena - Underrated, if you ask me. A huge help against Tate and Liza. It also generally kicks ass, with it's Intimidate, Swagger, Sand Attack and Shadow Ball (swap with Crunch before migration, obviously).
Diamond: Staraptor - Come on...it's just...brilliant. Brave Bird is my favourite Gen IV move...
Platinum: Absol - I've always loved it, but I slightly prefer Mightyena. Therefore, grabbing one on my way up Mount Coronet to prevent a disaster seems not only appropriate, but generally cool. It can also then hit Giratina with super-effective moves..
HeartGold: Politoed - I've loved it ever since I saw it's picture in the "pokemon annual 2003", despite never being able to obtain it until I got HeartGold, due to lack of trading pre-Platinum, as I only had one DS.