In almost every generation I have one starter I would definitely pick above all the others; Hoenn is the exception. I don't really like Mudkip that much; the design for Swampert, especially, just doesn't sit well with me for some reason. When I first played Sapphire I picked Torchic; as far as first stage goes, that one's my favourite hands-down. I thought Blaziken was kind of neat, but after two more generations have passed, I grew kind of tired of the Fire/Fighting type combo. (Still, Blaziken is my favourite of the Fire/Fighting starters.) I really like Treecko, but I don't like it nearly as much as most other Grass starters (except Turtwig).
So yeah, it will be a toss-up between Torchic and Treecko. On one hand, there are a few good Grass Pokemon in Hoenn already (like Roselia and Oddish - and Seedot, but I'm getting Alpha Sapphire, and Seedot was Ruby exclusive), but not many Fire-types that I like (IIRC, the only ones available sort-of-early on are Slugma, Torkoal, and Numel; Vulpix wasn't available until Mt. Pyre). On the other hand, I already have experience with Mega Blaziken thanks to its giveaway when X/Y came out, whereas Mega Sceptile/Swampert are completely new, and I'd rather have Medicham as my token Fighting-type. Right now I'm leaning a bit more towards Treecko, but who knows...