1. Vettel Challenges - GT5
2. Super License, Final Test - Sauber C9 on Nordschelife - GT4 (Japanese version, changed to Mercedes 190 DTM for US and PAL versions of the game, was not even close as a challenge)
3. International A License, Test 7 - TVR Griffith on Special Stage Route 11 - GT1
4. Super License, Test 2 - Ford GT40 on Seattle - GT2
5. Top Gear Events - Lotus Elise Race - GT5 (without ANY assists, that race is a 100% hardcore challenge)
Honorable mentions:
6. original (pre 1.02 patch) run with Gallardo on Toscana Night - GT5 - layout of the track was changed with the update and race became easy
7. Grand Canyon License Test with Subaru Rally Car - GT4
8. RUF RGT on Complex String - Time Trial event in Arcade Mode - GT3 (beat Kazunori Yamauchi's time)
However...
Single most demanding challenge in GT series history - on pair with the Vettel challenges:
Gran Turismo 5 Seasonal Events / Bonus Race No.2/ A-spec
Period of Availability: 24.12.2010. - 13.01.2011.
Mazda RX-7 Tsukuba Circuit / 10 Laps race
Without assists (all assists OFF, ABS OFF, no power steering), this particular challenge was the hardest challenge ever conceived in the history of GT series. Even with car tuned to max restrictions possible (Tyres:Max. Sports: Hard / Power: Max. 394BHP / Weight: Min. 800kg) it was a gargantuan challenge to keep the laps in sequence needed to catch the leader.
This particular event is the pinnacle of GT experience. Sadly, due to its extreme hardness, the outcry of the casual drivers was so loud (even here on GTPlanet, the words "broken", "impossible", "insane" were very usual these days..) that Polyphony decided to turn off the restrictions for all future Seasonal challenges in order to please the majority.
However, all races of Bonus Race #1 and Bonus Race #2 events will serve in memories as the Immaculate Challenges of the Gran Turismo series.