UAD Apollo is getting raves reviews. Anything by RME is a great choice. There are other high end boxes like Prism and Metric Halo. I'd probably avoid AVID hardware unless you plan to be a ProTools only person.
I'd avoid any interface that is OS dependent (Apogee) or DAW dependent.
For mixing stereo without summing, I'm very interested in the Dangerous Source.
For multitrack I'm thinking about the Apollo, hoping that the octo option will be announced at NAMM. The Yamaha 01v96i is another option I'm considering. For not much more than a RME Fireface UFX, it looks like it has much to offer. I currently use a Fireface 800 and it has been a great piece of gear but will likely be put to use in my live keyboard rig exclusively soon.