A long time ago it was Full Tilt as I loved the Daily Dollar tourney and their Sit and Go's.
After that it was pokerstars up until the end of 2015. I felt it was going downhill for the last year or so and all the changes at the end finally convinced me to move on (especially removing the $100k VIP freerolls).
Right now it's 888. I love the various freerolls and promotions and given that I mainly play micro stakes the lower traffic doesn't affect me much as I can find lots of games to satisfy me. It also seems like the weird and often times ridiculous beats I was seeing on PS happens a lot less here, more like they should statistically happen (It could be that it's just my perception, but I don't think so).
I've tryed many sites but in mine opinion pokerstars is the best. Nice softver support, tournaments, special offers, bonuses, freerolls. I just love it. For me its best site! Full tilt was good but now its for me joke.