Believe it or not, the best plug-in may already be on your system. Wierdly, a lot of people sing slightly flat (or sharp) when tracking via headphones. If you go to your pitch shifter and take your whole vocal up 10cents of one semitone, it adds life and 'air'. I discovered this from a book and it was like a veil being lifted from my eyes! I went back and did it to every vocal I'd recorded and it made a massive difference straight away.
Then it's down to eq'ing.
But I believe with the other guys that the top of your signal chain is massively important, and so a pre, and maybe a hardware compressor/limiter to make sure your signal is as hot and clean as possible will also really help.