I'll leave out MIM and the Jack... they are SO awesome, giant, prestigious, fun etc...and I'm not sure that's what you were going for originally...
so I'll have to chime in with my New England perspective...
Harpoon... July
very well run and cooker friendly - free beer, gorgeous setting, showers, water and electric, bands, Iron Man competition, state championship and an automatic for the Jack (at the moment). Fantastic festival. Just wish they would/could open it to more teams.
Lake Placid... July
this has got to be the most fun you can have on the 4th of July. very well run, extremely cooker friendly, all amenitites including continental breakfast and hot showers for the teams, an opportunity to make some money with the buck-a-bone contest, bands, fireworks, tons of stuff for the kids, state championship, and all at an historic and absolutely gorgeous setting.
and Pork In The Park... April (not really summer)
this was our first trek out of the NE back in '04, and we've been back ever since. cooker friendly, KUDOS to Sandy! one of the best run contests EVER, bands, vendors, kid's activities and rides, TONS of competition, great payouts, water and electric, and a state championship. Keeps getting better and better... just wish they could get some showers somehow.
I know you said three but I can't possibly leave out THE OINK...
Oinktoberfest... (end of September)
also well run, cooker-friendly, nice payouts, great bands, tons of kids stuff, beautiful area, goody bags (which are rare these days) and a state championship. And, don't forget the pumpkin cannon...
or Hudson Valley...
our first time going this year and I was super impressed. well run, cooker-friendly, beautiful place, showers AND a pool, kids stuff, bands, vendors, fun festival all around.
Oh man... I can't wait to start up again!!!