Right now it's Magic Bullet Colorista for AE (http://www.redgiantsoftware.com).

I'm also really partial to the Boris Continuum Complete 5 for AE(http://www.borisfx.com/product/continuum). So much good stuff in that suite.

What are your favorite plugins?

I havent got any new plug-ins for ages but I was in love with the trapcode package when I had a working copy of After Effects