Depending what are your tasks and plans for poker.
If you want to learn poker or you were on a long break I'll suggest ACR.(Freerolls every hour)
If you aim to get few bucks(10$-20$) to try
real money games, BOL is the way to go.(Freerolls with delay of 2-3 hours)
If u want some bigger cashes from freerolls, try PS Poker School Leaderboards.(Freerolls every 4 hours or even more rare, also you want to know this is time consuming and you can meet here really strong players, more than anywhere).
As you can observe, I didn't say a thing about one of the biggest rooms, 888poker. I played here but I didn't see any kind of advantages over other rooms.