For my Sapphire game (that mysteriously vanished) I chose Mudkip first, but restarted when my Swampert was in the 60's because I was bored of it. Then I chose Torchic, because I HATE HATE HATE Sceptile.
On Emerald I started with Treecko, played through 'til he was in the high forties and restarted for Mudkip. I'm about to re-re-restart my game, though, and I'm going to pick Treecko and keep him a Grovyle.
On Leaf Green I picked Squirtle first because Blastoise is a beast (and this particular one was my first lv.100 :D). Then I went through with Bulbasaur, because I really strongly dislike Charizard. But my Venusaur disappoints me, so she's sitting pretty at lv.68 in a box. Then I went to Charmander reluctantly, and now have a very hated level 95 female Charizard. >.<
/long-winded post