Hope you don't mind IronBot, but I've gone ahead and added the other three starters, since I'm sure more than a few people have one of them as their favorite.
As for myself, I went with Treecko, with Piplup being a close second (if we were going with final evolutions, that'd be reversed). While I'm not a big fan of starter Pokemon, my Treecko from Sapphire is the Pokemon that I've had around the longest, and it's currently living on my HeartGold save. Hard to believe that I've had it for 11+ years, but I'd be devastated to lose it, even if it's just pixels and data, haha.