My favorites and most used? It depends on purpose and some of these are definitely not for purists who are only cleaning up photos for realism and superb printing.
That said my faves are:
* Grain Surgery for noise reduction, smooting, and adding grain
* Optik Verve's "Virtual Photographer" for mixing photo effects
* Filters Unlimited 2.0 with the free Photo Aging plugin
* Flaming Pear's "Mr. Contrast", "Melancholitron", and others
* Xenofex 2 suite
* Eye Candy suite has some nice filters mixed in with some useless ones
If I could only pic 3 it'd be Grain Surgery, Filters Unlimited, and the Flaming Pears.
