Wow. That's a pretty great dilemma to have. You really can't go wrong with any of those.
The Cohos Trail should probably be the third in the list, although it's still pretty amazing. It would be a toss-up between Maine and the LT for me. I'm partial to Maine just because I'm from there, but the Long Trail is incredible, too.
Maine has a lot more lakes and ponds with some really great lake-side campsites. The above-tree-line sections of the AT in Maine are also the most spectacular places you'll see anywhere on the east coast.
The Long Trail is rugged and glorious, and once you get north of the AT you're going to see very few people. Once you get north of Stowe and Mt Mansfield, you'll be in a really wonderful wild area. There are also a huge number of really fantastic towns you can get to from the trail.
If you've hiked in Vermont a lot already, I'd say do the Maine AT this year, and the LT the next. Maybe the Cohos the year after that.