There is no easy answer to that question. It really depends on your personal preferences and priorities. There are numerous factors that can sway you from one poker room to another. I've listed some of the most important factors to consider:
1) What level of rakeback am I earning?
Some sites don't offer any rakeback, instead focusing on Frequent Player Points and rewards. The sites that do offer rakeback vary from 27%-40%.
2) What bonus do I earn from depositing on their site?
Many sites offer enticing sounding matching bonuses, but often the more important factor is at what rate do you clear your bonus and how long do you have to clear it.
3) What game and level do I prefer playing?
Poker rooms offer different games and may emphasize tournaments, SNG's or cash games.
4) Are you a multi-tabler?
Some sites allow you to play more tables than others.
5) What type of fields and table options do you prefer?
Some sites have massive player volume. They have huge fields for their tournaments and large prize pools, but you have to constantly navigate through huge fields. Other sites have smaller player volume so you can't table select as well, but you are playing in smaller tourney fields and can become familiar with the player base.
6) Do you use a HUD (heads up display)?
Some sites don't let you use a HUD.
7) Do you play to other games than poker?
Some sites offer sports betting, casino, backgammon, rummy, and even bingo. These can be additional entertainment options, or they may influence the quality of players who come over to play poker.
8) Is the site lucky for me or not?
As frivilous as it sounds, some players switch sites because they run particularly poorly on another site and need a change. With a long enough sample, it should all even out, but in the short term you may run better on one site than another.
9) Who else plays there?
Many players prefer to play on sites where their friends play, for the social aspect. Additionally, the sport of railing top players has become increasingly popular, so many people will watch the highest stakes games on sites like Full Tilt or PokerStars.