This is a question almost every poker player will be asking at some point of their career. Unfortunately there is no simple answer for this question.
When you are choosing a room, there are many factors you want to consider. Traffic, reliability, software, HUD allowed or not, rake, rakeback and so on…
So, when you are looking for a new poker room it takes a lot of Googling and making rake comparisons. The traffic info can be easily found here: http://www.pokerscout.com/
What room is best for you depends a lot on what do you need or want. For example, is good and reliable software more important than having a bit better rakeback? Or is playing without a HUD absolute no for you?
For me the best room is PokerStars. I enjoy the high traffic, being able to use a HUD, good software, good support, fast withdrawals and so on.. Only negative side is the low rakeback. But because my volume is on the lower side it doesn’t matter that much. PartyPoker would be another decent option but since the rake there is a bit higher at NL25 and lower, it really doesn’t make that big of a difference. And since a lot of Stars hardcore grinders are moving to Party, Stars player pool has become slightly softer. My second pick would probably be a room in GG Network (Natural8 for example), because the player pool is super soft and rakeback is great (or at least was at N8). HUD is not allowed there but it doesn’t trouble me that much. But what does is that the software is horrible. Animations lag and for some reason playing 2 tables makes my CPU load double compared to playing 2 tables at any other poker room.
So as you can see, there pretty much is no “perfect” room. You have to make compromises. Choosing the perfect room for you takes some research and depends a lot on your preferences.