I look for racks that are sturdy, made out of material that will last (plastic / metal), tubs slide out easy, tubs have proper clearance, and tubs are easy to find if I need to replace them.
I have RBI plastic racks and a Freedom Breeder style rack. I like the plastic racks, because they hold humidity very well. Personally, I like the metal racks better. I plan to purchase Freedom Breeder or ARS racks in the future.