Anything grass type

a gun?

I mean if we're purely talking match up for starters? Gotta go with Turtwig. The whole line gets access to ground and rock TMs for coverage. Provided you get the TM, even just Turtwig can learn sandtomb and mud shot. You could do some serious damage in the first two evos with that.

By the numbers on the last Evo, Charizard is set up as a Sp. Attack sweeper but it's P. def is trashed. Torterra is set up opposite with very solid Physical Stats, low specials, lower speed and solid HP. They're pretty asymmetric. There's a world where you switch in Torterra to eat a Charizard fire attack, queue up rockslide and hold your breath that it doesn't have a special flying move in the pocket that can chew up the rest of you.

Plus, you can use it to set with stealthrock so it might not even be a bad option for leading the party given that it's beefy enough to eat a smack or two.

Squirtle

Use mudkip. Swampert might be the best starter evo of them all. Not sure if it can survive a 1v1 vs Venusaur though. If it's just 1v1, you might want to try Infernape since it has a huge movepool and hits hard and fast.

Fennekin because it's adorable and the best. Don't care about anything else beyond that.

Eevee->Sylveon is my pick. Fairy type is OP, and pixellate + hyper beam/voice will sweep just about everything.

Litten->Incineroar is probably the best tho

A Fire-Fighting starter, can't remember it's name but I heard it's pretty good :^)