Charizard is my favorite due to being the first one I ever got (FireRed was my first game), and because of how good it is in recent games. Charizard Y is definitely one of my favorite Pokemon to use in ORAS. Meganium is my second favorite. Typhlosion was my first Gen II starter, but I've used Meganium more commonly in most playthroughs for some reason. Swampert is my favorite Hoenn starter and also the first Hoenn starter I ever used, Empoleon is my favorite Sinnoh starter and also the first Sinnoh starter I used, Samurott was my first Unova starter, and Chesnaught was my first/favorite Kalos starter.
I don't feel like listing all #18 in any specific order, so instead I'll rank them by Generation (in a chain):
Johto >= Kanto > Hoenn >~ Sinnoh >> Kalos >~ Unova