It varies, usually any starter pokemon I get is something I stick with.
Preferably my Venusaur from Leaf Green.
Yet, I could also lean towards my Samurott and Empoleon.
I always pick water type of whatever game I'm playing. My favourite to use is Piplup/Prinplup/Empoleon but I've also had Squirtle, Totodile, Mudkip, and even Oshawott
I have one for the three Pokemon games I play, for emerald I really like Blaziken, for HeartGold I always use my Scizor, and for Diamond me loves some Honchcrow.