BetOnline is probably top of the list except for right now I'm not using it. I'm using MyBookie and Bookmaker both for different reasons. MyBookie because of them sponsoring the March Madness pool here at BMR and also because I have to complete rollover. I will say that I have enjoyed my experience at MyBookie and didn't have a problem receiving a payout from them which is my number 1 priority when choosing to play at a new sports book because after all it sucks if you have to worry about getting paid. Both MyBookie and Bookmaker meet that criteria but to me MyBookie is a rec book that I probably wouldn't recommend ahead of the Big Three of Bookmaker, BetOnline and Heritage.
As for my personal favorites that I currently use or have used in the past I'd probably rank them Bookmaker, BetOnline, Heritage, Bet105 and BetUS in that particular order. Bookmaker is number 1 for me because even though I'm no pro they take whatever bet no matter how big or how small it is and I just have always liked and respected them because of that. BetOnline is a close second for me because they're similar in that they take big bets and small bets no matter what and they probably have the quickest payouts out of all the books I've tried. The last three I think is a matter of personal preference because they're all very good outs to have in their own way.