They all definitely had their moments, with Chandler being the best. But he got more boring after him and Monica got serious. Phoebe and Monica can be over the top sometimes, but extremely funny other times. Ross and Joey ain't all that, but they have their moments. So this is my list:

Chandler
Rachel
Phoebe
Monica
Ross
Joey

I just think Joey was a bit exaggerated at times. I know Phoebe was too, but she got really awesome in the later seasons. But by all means, everyone is really funny, so there's no shame in being last on my list.