Before you get the <INSERT BRAND NAME HERE> system, what do you use now and do you like it?
You mention Partner, so I'd guess than an ACS 6.0 is pretty stable and if you're familiar with it and/or have parts for it, then that's the brand I'd go with.
Any properly installed system of the 6x16 size is going to be an install and forget about it system. I'd lean toward a system that has a little bit of overhead and one that is tried and true. That tends to mean the top 4 brands, but not exclusively. Again, this is a common system, so something you can drop in a new cabinet or add a phone would be adequate.
The top 4 brands for key systems? In Alphabetical order Avaya, NEC, Nortel, Panasonic. Of those, I'd look carefully at the NEC DSX before I plunked down my money, but the other 3 pretty much a Partner, MICS, or TA/TDA ought to be fine. I'm not bashing the DSX, but there's not enough history for my forgetaboutit rating.
LkEErie