My favorite is Bulbasaur because Venusaur can make a very fun tank, although, Meganium has better defensive stats. If they only had Thick Fat...
My second favorite is Charmander, Charizard has a very useful physical sweeping movepool. If only Charizard had his Attack and Special Attack base stats swaped, Charizard would be a lot more useful.
My third favorite is Chimchar, Infernape is very useful and can kill the other 2 starters of its generation. Although I don't know why they made another Fire/Fighting starter, Blaziken is now outclassed by Infernape. Sad.