Charmander, by far.
Before last year, I only used Squirtle as a starter in Pokemon Blue. He seemed well rounded to me, I liked water Pokemon, plus I always wished I had one when I had to face Brock in Pokemon Yellow when I was younger. I used Bulbasaur as well, but quit with him halfways through (just wasn't thrilled). I picked Charmander one day, being bored out of my mind, and I never regretted it. He evolves into Charizard, one of the strongest and fastest Pokemon of all time. Because of my runthrough with him, he is now tied with Pikachu (my first ever starter) as my all time favorite Pokemon. His stats are epic. Sure, Brock and Misty give him a hard time, but afterwards, it gets easier.
Charmander, plain and simple.
Kanto, Johto, Hoenn, and Unova Champion
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