Spa, Suzuka and Indy for me, in that order, and here's why:
Spa: Do I
really have to explain this one? It's really the last circuit left on the calendar (assuming it's back for '07 *crosses fingers and toes*) that's B.O.O.: Brass Ones Only!
Suzuka: The crossover, the Snake, Spoon Curve, 130R...what's not to like? It'd be a shame if the Japanese GP moves to the overgrown kart track Fuji has been turned into.
Indianapolis: I just had to include this place, and not just because I'm American. Yes, the infield might as well have been laid out by a schizophrenic hamster, but it's heaven for spectators, forces everyone to run a compromise setup and, IIRC, it's the oldest purpose-built motor racing facility still operating on the planet.
Honorable mentions:
Monza: Closest thing to an oval on the tour, but that reminds me: BRING BACK THE BANKING, DAMMIT!!!
Monaco: As one driver whose name escapes me so eloquently put it, "It's like flying a helicopter in a phone booth." 'Nuff said.