Never did like Gengar, never could get into him (by which I mean I could never get him). He was too much of a little imp.
I do <3 Mew. I actually watched that movie in a theater and got one of those Ancient Mew cards. Dunno where it went, but it counts for something. And I never could get him in the game, but I remember getting a perfect picture of him in Pokemon Snap (yeah, that game I remember. I like it, as I recall).
And while Butterfree wasn't the most powerful of mons, I actually tended to have one as a key party member for a good chunk of the start of the game. Gives you some good grass attacks, speed, and (most importantly) sleeping powder. I eventually needed to box him when things got tough, but I liked having the little guy around for a while.