All three are good. Bulbasaur is the only dual-typed starter, being grass/poison. This makes for a good advantage while going through the Viridian Forest, and later against Koga. Bulbasaur is also strong against the first two gym leaders. Pick it if you want an easier time starting out.
Charmander is weak against the first two gym leaders and doesn't become dual typed until Charizard, and even then the typing isn't too good. However, fire types in general make a good offense. Also, in FR/LG it is easy to get it to level 13 in time for Brock, at which time it will learn metal claw. This can sweep Brock's team.
Squirtle remains water type through all evolutions. It's well balanced. Personally I don't see this starter picked as much as the other two. It'll be strong against Brock and neutral with Misty.
So pick whichever appeals to you. They're all decent pokemon.