I had to choose Cyndaquil as my favorite starter. I remember the first day I got Pokemon Gold. I tossed the cartridge into my Gameboy Color, flipped the POWER switch, plugged in my headphones, and got carried away to the Johto region. My first choice was, as in Pokemon Red/Blue, the badass Fire-type Pokemon. I grabbed a Cyndaquil, ran off to Cherrygrove, ran back to New Bark, gave the Egg to Elm, then left (accidentally forgetting to say goodbye to my mother). I remember going on a Mono-Pokemon challenge with every Pokemon game I had, until I realized that I should probably be training other Pokemon than my Starter. Hell, sometimes, I would just leave my Starter Pokemon in a PC Box, grab a Ratatta or Pidgey, and just go through the entire game with that one Pokemon, evolving it whenever it reached it's level of evolution, teaching it whatever TM I needed to to get past a trainer that was beating me.
I have great memories of going on Mono-Pokemon challenges with just my Raticate, going against Chuck and constantly getting stomped by his Poliwrath's Mind Reader+DynamicPunch combination. Or surfing through the ocean, using my Typhlosion to battle off all the Tentacool and Tentacruel that came our way. Good times.