Bulbasaur has type advantage against the first few gyms but has disadvantage against later gyms. By later gyms you’ll have other Pokémon with various types.

Bulbasaur has a strong type advantage against the first two gyms, making your start easier, and you'll have a type advantage for a chunk of the first two members of the Elite 4. It can also be a pretty scary 'mon to face with Razor Leaf, because Gen 1 is held together with prayer and those frequent crits can hurt.

It's also cute.

I've seen some people who like it because it gets the Powder line of moves(Poison, Stun, and Sleep) so it's easier to set-up a catch, but I personally prefer a Paras with Spore for that.

I usually chose Charmander and let me tell you; the early game is NOT fun with a fire-type as your main. Ember might be a cool move while you're still roasting bug-types along the way to the first gym, but Brock will kick your butt unless you grossly over-level or get a different Pokémon as your main damage dealer, as fire-types suck against rock-types. And guess what the super long next tunnel section is filled with ...

Seriously, as much as I love Charmander and its evolutions, Bulbasaur is the better choice for a first experience.