the black commercial flir thermosight units are ok but not the best if we're talking their commercial line. Flir government is a whole other thing but you're talking everything north of 10 grand a piece. I'd also recommend having a handheld spotter as well as the scope depending on your style of hunting. For me, I ride around in a ranger and scan field after field rather than sit in a blind. That's really where the thermal advantage is. If you're hunting from a blind then you really don't get the main benefit of thermals. Most any night scope is ok for that. I also use my handhelds about 10 times as much as my rifle scope.

It all really depends on your budget. The best options out there right now are in the 6-8K price range (IR defense, high end armasight 640 units, etc.)

Sensor and lens size are the big factors but display is a factor too. Large OLED displays are very much preferable.