Tourists aka "Space Flight Participants":
Probably Dennis Tito; if full flight duration is counted, then a tie with Hazzaa al Mansoori
The source says "6 days aboard ISS" for Tito and "8 days aboard ISS" for al Mansoori; the chart apparently is for the full mission. The difference probably reflects the short rendezvous profiles currently in use. If one goes by that, it would be Tito.
*nauts:
Pedro Duque had a short stay (7 days total) I think (up with 7S on 10/20/03, down with 6S on 10/27/03), he may be the shortest stay-er ever.
Talgat A. Musabayev and Yuri M. Baturin (labeled "Russian politician") stayed the same amount of time as Tito.
Yuri Shargin: 10 days total in 2004
Roberto Vittori: 10 days total in 2005
Sheikh Muszaphar Shukor: 10 days total in 2007
There were a number of other early Soyuz swapout "taxi" flights with 10 day durations; I have not listed all those crewmembers.
André Kuipers: 11 days total in 2004
Sources:
https://www.nasa.gov/feature/space-station-20th-space-flight-participants
"ISS As Flown Flight Plan" - personal notes.