By virtue of typing people think Bulbasaur is easy mode. I never really found that to be the case. Vine Whip is learned at level 13 unfortunately and only has 10 PP (note: level 13 applies in RGB). Not to mention there are a lot of Poison types in this game as
>>33429147 mentioned. Even in FRLG I used Venusaur for a while, but it became less usable later. Not that it is bad by any means, but other Pokemon seemed to have done its job better. I think Sleep Powder is some high level 40s attack for Venusaur in Gen 1 games, I forgot. Almost always critting Razor Leaf is great though in RGB, and earlier evolution (level 32 vs level 36 for the other two) is also an edge it has over the other two. Unfortunately, it cannot learn Earthquake until FRLG (if you are playing that), and fighting things with 4x resistance to Grass is more common than not in this game.
I found Squirtle to really be the most versatile here. Misty is not much of a problem with the Mega Punch TM for coverage (or Bite in FRLG and that is a lot better), Lt. Surge can be covered by the Digletts or if you have a Sandshrew, or maybe even Grass mons. Erika's Grass types can fold to either a bird or a Fire type which you can catch in Route 8 (Vulpix in LG gets special mention for early Flamethrower and Safeguard). In terms of effectiveness I found Blastoise to be quite satisfactory in movepool and pretty balanced stats. You could teach it Earthquake if you would like, and Ice Beam or Blizzard (note: Blizzard had 90% accuracy in RGBY) can be of use to hit Grass types hard. And I was always the water guy so it was fun. Charizard just hits hard with Flamethrower generally, but it is a little tough raising it. Its natural learnset is not too good in RGB though.
But really,
just pick your favourite. 
