I only know of 1 decent online pool game, its called "3D Ultra Cool Pool" , its not realistic but the physics are pretty good, and theres always loads online to play with.
For offline pool/snooker games, Paradise Pool is one of my favourties as is the World Championship Snooker series, that is the most realistic game ive played full stop! Its brutally hard and you have to really get into it, because if you miss a ball, you could be in for a long wait!