Full Tilt is pretty good, and since its now linked to Pokerstars you can send money between the two freely which could be handy if you bust your roll on one, instead of depositing again you could just send yourself money from the other one.
I have always found the Full Tilt software quite nice and I always feel that i get rivered a lot less there although thats probably just my imagination.
PKR is good fun and you can view peoples results if they have chosen not to hide them, so if you cannot see on sharkscope for example, they may still be viewable on their PKR profile.
The downside to PKR is that the fields and payouts are much smaller than on Pokerstars/Full Tilt. Although this can be a good thing as it means the tournaments are shorter.
Partypoker has some big tournaments like Stars and Tilt, but i have never really liked it there. I dont know why. It just never feels like a nice experience when i play there.
I have played others but these are the main ones i can give my opinions on.
The best thing to do is to see where you cna get the best deposit bonuses or rakeback and go through one of the sites that can provide you with these options. Sites such as 2+2 and Pocket5's.
Hope this is of some use to you.