When flying in uncontrolled airspace, which ATC frequency should I contact if I have an emergency?
The safest approach is to use 121.5 until someone instructs you to try another frequency.  ATC will monitor 121.5 at all locations it has antennas at. Additionally, military, some airlines, some medvac, and other operations will monitor 121.5  So your coverage will be greatest if you just use 121.5, unless you are already talking to ATC.

Addendum: Really in uncontrolled airspace?    If you are really in uncontrolled airspace, in the CONUS, you are likely flying rather low.  121.5 will be your best best in that case as well.  With enough time, you might work out a relay with higher traffic, perhaps on a center frequency.  But 121.5 remains the go to frequency.