I don't know of any, but maybe someone here does. I do know one thing, though: everybody will have a different "best" system. Even if you were to ask here, you would get 10 different answers! I suggest you search around the internet for different systems, look at their features, the company's reputtaion, and availability of parts, and then make your own decision.
....that aside, I would stay away from less known companies such as Bizfon or Win and stick with big ones such as Avaya, Nortel, NEC, Panasonic, Comdial, etc.
