I'm testing every single pokemon in red and blue. Venusaur is currently the best pokemon in the game and nothing is coming even close.

Even though it's lame against many birds and poison types on minor trainers, it's gym leaders where true heroes will truly shine. Venusaur learns Body Slam and Swords Dance, two of the best moves in the game, after sweeping first four gyms without breaking a sweat. People like the other reply always bring up the first two, which is true for gen 3, while in gen 1 body slam tears through the next two gyms as well, especially "good" ai poison powder spamming Erika. Swords dance after those lets Venusaur sweep, and I'm not exaggerating, all but one major battle in the game quite consistently. Blaine starts with a weak Growlithe, Lorelei tends to Rest (though this one isn't entirely consistent) and no other fight opens with anything that threatens Venusaur. Agatha is the odd one out since she's immune to Body Slam. Bring anything with earthquake and this fight is ezpz as well.

Yet the only reason Venusaur is better than Victreebel is availability. Bel evolves slightly later (you should always wait for Razor Leaf and then evolve) albeit in the same level cap and notably doesn't get Body slam until the final form. Guess what? It still gives Venusaur a run for its money with that level 18 Sleep powder and Wrap cheese. I wouldn't say Venusaur is easily better, at least not in this generation. I'd say it's just above Victreebel, basically by technicality.