I think GGPoker and Pokerstar have good clients but there are many good and professional players on these sites. Other good sites with softer players are Unibet, ACR, 888Poker.
I actually preffer Pokerstars for its layout. I've tried other poker sites and I felt that it wasn't the same. It's reall ypersonal, but since I just play for fun I'd rather play where I find it easier to navigate and am already familiarised with.
my favorite poker room is poker star. even if the level of the players is generally quite high. I got used to the graphics and I feel comfortable and relaxed when I play in this room.