Oddish, but i dont know why. I've liked him since gen 1, I remember going nuts when I found out he was going to be released in the Jungle Series booster packs.
I think it's because I thought he was cool in gen 1 but I had Red version and you couldn't catch him in Red only Blue. He was unobtainable to me until my friend/brother picked up blue a few weeks or months later and then had to wait until one of us got the trade cable. That is the only explanation of why he is my favorite.
To this day every new gen I buy I make sure to get the one Oddish is in and I never evolve him, sometimes I'll catch a second one to get to Vileplume but my first one always stays an Oddish.
Salamence. Love its design, love its playstyle, love its story, love its raw power. I have a ton of history with this Pokemon spanning multiple different Salamences, and I can still remember how elated I was to finally get ahold of a Bagon back in Ruby.