If talking "crewed" (aka "manned") aircraft, using propellers:
The Grob Strato 2C was a German experimental high altitude research aircraft. Powered by two turbocharged piston engines (dual PUSHER propellers) and featuring an extremely long span wing of composite construction, one aircraft was built in the 1990s, but was abandoned despite setting a world altitude record for piston-engined aircraft on its last flight ("crewed" record, not over-all)
1995, August 4: 60,897 ft / 18,561 m
Note: Unpressurised cabin record is the Caproni Ca.161: 1938, October 22: 56,850 ft / 17,330 m
The theoretical "max altitude" a propeller would remain effective is "the stratopause (formerly misspeak, 48,000 to 55,000 m above sea level)" ...the level of the atmosphere which is the boundary between two layers: the stratosphere and the mesosphere. In the stratosphere, the temperature increases with altitude, and the stratopause is the region where a maximum in the temperature occurs.
IMHO: Past this region there is not enough particle density for practical propeller use. This, approx. 50,000m altitude, is "possible" but exceeds any existing technology, at least on designs I have seen (like the previously mentioned NASA Helios's 29,524m)