As a man who has tried them all personally the Fire Pea is actually really good(I I masted him in rank already) I've seen people using the Toxic due to that aoe specially against Imps and also noticed a lot of Plasma Peas so im assuming they've gotten better.
To be honest, I haven't seen a stand out. I've seen plenty of people with the stock Pea Shooter who were/are really good, and plenty with fire/toxic/plasma who just weren't. So far, I would say, they seem to have a really good balance between the sub-types for this particular class and would advise using which ever you end up enjoying the most, as I think it's going to come down to personal preference more than which one is really "the best".
