My favorite (as if everyone didn't already know) is the Doctor, aka the EMH. He was consistently great, and made even the mediocre episodes of VOY worth watching.
My second favorite is Bones, for his endearingly surly ways and all-around awesomeness.
I guess he would be followed by Bashir. Which is weird; normally I'm not crazy about characters like him, but I really like Bashir. He's annoyingly eager, but sweet.
As for my least favorites, it's a three-way tie between Crusher, Pulaski, and Phlox. Phlox might have been higher on my list, if it weren't for "Dear Doctor". Stupid