I'm not a pro, but I'd like to chime in on this one.
For my voice, it's the API-2500 VST, hands down. It puts the "big bottom" on my voice. Who knew I had such a skinny bottom and needed to fatten it up?
My second choice is the "Fishfillet Blockfish" compressor, the free VST from Digital Fish Phones. Perfect when gooey-colored compression is desired. Can't beat the price, either.