Ultimate Adventures (Milton), Area 51 (Toronto), Flag Raiders (Cambridge)
This is the field I learned to play on so I'm a little biased when I say that it is probably one of the best fields out there.
The layout is cool and very urban-combat-like. It takes some getting used to at first and can be confusing to newcomers. Whenever I've played it has been relatively clean (not as bad as other fields... cough ... Area 51 ... cough...)
The only drawback is the staff. Some of them are okay, although I found them very rude and sarcastic. I was there at least once a month for a year and they still treated me like trash. The only reason I go back is because the field is so much fun to play.
The only problem with the gameplay is that the field never changes. For players who like to play a lot it gets boring when you play the exact same scenarios all the time. (although crossover is always fun)