I'm no expert, but the word on the street is that it ain't what you use it's how you use it. HAVING SAID THAT, I recently got my hands on a couple of really useful tools. Waves vocal rider speeds up vocal production enormously, and their CLA vocal plugin is very tasty. Likewise their Rverb.
I use logic, so I don't know what comes with Pro Tools, but I do know that Warren Huart speaks highly of the compressor that comes with it.
Bill's right - collecting plugins is expensive. I'd get to know the packaged ones well enough first to get a feel for what's not working for you before putting a list together. You might find you don't need anything, or that your needs change as you go along.