If i had a way to use Ivysaur from gen 1 over Venusaur I would. The move pool cannot reliably support all that well and you trade your offensive stats when you pick up eviolite for defenses. Once you have the defenses you need a way to use it, which ivysaur does not, outside maybe like stun spore / sleep powder ect... which have low accuracy.
So i will just say wartortle just because i can actually use it and i like Bulbasaur, Ivysaur and Wartortle, all more then the starters and evolutions i did not mention.
If i had to pick a runner up then either vaporeon or magneton.