Squirtle. He's got bulk, versatility, and (reasonable) power. Not too many replacements. Gyarados is really good but evolves from Magikarp of all things and has bad Special Attack, Staryu comes mid-game, Lapras comes at too low a level, and Vaporeon is good but comes at the opportunity cost of acquiring Jolteon (IMO) the best eon instead.

Bulbasaur would be second. Good in the early game, not too good in the mid-game, average in the endgame. Sunny Day + Solarbeam can blow Lorelei and Bruno straight up though, combined with Sleep Powder. Exeggutor, Victreebel, and Vileplume are all suitable replacements.

The worst is Charmander. Fire is just kind of awful in Kanto lol. If you want a Fire-type, Ninetales, Arcanine, and Magmar are all better IMO.