I think it's going to depend on:
A. the sport involved
B. the amounts you typically play
C. Your patterns of play -- early/late betting, scalp/middle, etc
D. banking methods
Probably other factors as well.
That said:
1. Pinnacle (without a doubt)
2. Skybook (for hoops, unless you are a multiple dime bettor only)
3. Hollywood (reduced juice always, mostly 3K limits, some different numbers)
4. Olympic (early numbers, different numbers, high limits, good banking)
But I'd have a hard time limiting myself to just 4.