The Paintball Arena (Kitchener), Area 51 (Toronto), Flag Raiders (Cambridge)
This was the first outdoor field I ever played on and is one of the best. The fields are great for newbies and they have a speedball field which gives older/more experienced players that taste of tourney action they've always wanted to try.
I haven't played there in a while, but the last time I went it was awesome. The ghost town is a lot of fun, but was somewhat dilapidated (although I heard they planned on fixing it up).
They have excellent group rates and the staff is friendly. The BBQ lunch wasn't gourmet, but was adequate to fill our needs (mmmmmm... hot dogs...). I definately plan on going back there this summer at least once.