IMO zombies pros lie in their more accurate fire and bulk, while plants (except for the cactus) trade accuracy for damage and less HP.
I like playing plants like the the cacti and sunflowers because of how accurate their bullets feel, on the other hand kernel corn or the peashooter bullets travel speed feel really awkward.
Now to answer your question, on the zombie side I love playing All stars and scientists. I choose the moto all stars because it can effectively make an enemy completely useless with his mud bullets. And on the scientist side I like picking Dr.Toxic as I think it's the more offensive variant and it can still provide a ton of healing with his skills.