The longest distance over which I have received a beacon so far is OZN Prins Christian Sund with 3300 km. A bit of research on the internet made me think that this beacon might very well be operated by Tusass Greenland. Apparently that was correct: Bo Mogensen, who also confirmed my Aasiaat Radio reception, sent me a brief email as QSL OZN-372 Prins Christian Sund.
The OZN radio beacon is located at weather station at the Prins Christian Sund on the southern tip of Greenland. This station was founded by the US during WWII under the name Bluie East One.