So far I've done the San Marino GP, Monoco twice, French GP at Magny Cours, and the late, great Phoenix GP. Phoenix was the best!- only about ten people there, the temperature never went over 110, and we could see a full 20' of track!
Monoco is the most amazing GP- you can walk right into the paddock in the evening after practise & qualifying, chat with mechanics in the bars in the evening, watching a single corner taken at surreal speeds from only about 30' away, watch the drivers cruise in borrowed Ferrari's after their qualifying sessions (at least I saw Michael Andretti do this, wasn't quite as cool as seeing one of the 'real' F1 drivers do it

). I did it as a poverty-stricken fanatic both times, sleeping on the beach in nearby France, and it was a lifetime-memorable event.
As a spectator, you can't really get that much of a sense of the event, so passing doesn't matter as much as it does on TV. What you really experience is the ambiance, the sound, the sense of mind-bending speed. For this reason I would choose an ambiance track (San Marino is fantastic) over a passing track (Magny Cours sucks-too flat and out-of-the-way). Places like San Marino are magic all weekend, going to fantastic restaurants filled with tifosi, etc. Plus, you get experiences like this: after the '89 San Marino GP, I was hanging out at the exit from the paddock area, waiting to see who I could see, just soaking it all in. Everybody was watching for limosines or Ferraris, looking for drivers, so this little guy exited on a Marlboro scooter almost completely ignored. I had Ayrton Senna look directly at me, unnoticed except by me and the old Italian guy next to me, give us a wink & a smile, and take off. The old guy and I look at each other, then around at evrybody else still anxiously searching the paddock, and we both break out laughing. Tell me anything on Speedvision compares