If you are going to use only one then PinnacleSports.com. They have lowest mark up and most of the time high limits. Multiple max bets are allowed too. If are going to use many look at Bet365 (have accumulator promos can be used to get reduced juice) - and keep an eye on Ladbrokes and William Hill for occasional off market prices, good value on props and alternate lines (takes some selection).
When talking NBA specifically then adding in Asian sites like SBOBet and 188Bet is smart too.
For US sports, I place almost all my bets with Matchbook and Pinnacle.
Closer to game time you will mostly do better at Matchbook than Pinnacle. If you are looking to take early lines and odds then Pinnacle is the place as it takes a while for the liquidity to build up at Matchbook.
The Greek, SBO and Betfair are also useful to have.
I avoid using any books which serve the US market as I consider them liable to attack by the US government with possible loss of player funds.
For NBA you can get some great odds from Paddy Power, Bet365 and Boylesports. These are all great, as long as you manage to keep hold of your account before being limited or closed. I have also started using Marathonbet, and have seen some great odds available.
YouWin has good odds, but I would not play there, as they have a bit of a bad history in the treatment of players.