Well, let's look at a quick rundown
Treecko - Awesome, fast, great to use just because of pure badassness. However, it needs to go through a line of breeding to be truly great.
Mudkip - I don't have a lot of experience with it, but from what I can tell it's on the more powerful side, only one weakness which can be covered.
Torchic - The original Fire/Fighting, plus it's an Uber! Need I say more?
Overall I like Torchic the best. Great design, original typing, awesome moves... Although all of them are really good.