Pokerstars easily has the best tournaments. Good format, lots of options, times, games, etc. Pokerstars also has soft games and has the best support around.

Now, lest I sound like shill, I don't play PS much, I usually play where I have a great rakeback which is at UB. I like UB, there's usually decent games, but it's a smaller site and game quality varies.

Party is big, soft games, and total crap support. I hate their tournament formats, too short stacked at the start.

I play UB or Stars. Party's support sucks and their "cheating" detection is flawed. They sent me an email complaining I was running a tool I've never owned or even heard of (Poker prophet or some such), there was no way I was risking my money there again since their detection software clearly has issues.