My faves are Frontiers, MTM (R.I.P.), Kivel, the excellent Long Island and the aforementioned Atlantic. AOR Heaven releases are quite good too (certain genres).
Adding Escape Music to the list too, I'm suprised no one mention them. Based partially on the now defunct Long Island Records, Escape Music's reissues (in particular) are a joy to the ear and a pleasure for the eyes. I'm talkin' 'bout the double/triple cds with bonus tracks and excellent digipack packaging.
Z Records are worth a mention but I can't stand the politics surrounding the label.
Thumbs up also for Rock Candy and Wounded Bird for bringing us those reissues.