Squirtle, mostly because it is the most nostalgic as being my first Pokemon ever, but also because I find it to be the cutest, and has the best designed evolutionary line. Not only that, but I like defensive pokemon and it's the most defensive of the original starters.
Also, I think that charmander's family are overated, the designs are unoriginal, and the pokemon just aren't that fun to play with.
Bulbasaur I think is cute, useful, and definitely has the best competitive final form (excluding megas), however it got outclassed for me by many other grass/poison and bug/poison Pokemon. I do have a soft spot for it though.