A few that I have experience with:
WSEX is the best IMO. Their software eats so little data that if they were in Africa they'd be getting humanitarian aid.
Skybook/IIS is very good in that they leave the lines up very late, sometimes up to 2 min. after the start of the game.
Oly, Hollywood and CRIS have been quite reliable over the years for me.
Pinnacle is sychronized to the atomic clock which makes them reliable but when the lines are moving fast it's almost impossible to get a bet in because it keeps getting rejected.
WWTS gobbles a lot of data and sometimes has processing delays at crunch times.
5dimes has some serious delay issues at crunch times ... sometimes the software won't even load.