Douglas is the largest town. The main TT compound is there, to include the start/finish line, the hot pits, the main grandstands, the VIP compound, the Monster Energy show, yada, yada. The Promenade and Bushy's Beer Tent is there too. Douglas is for the younger party animals.
Peel is a nice quiet little town, with Peel Harbor, Peel Castle, a grocery store, and just enough pubs. We stayed in Peel. It's great for grownups that like a little peace and quiet. Peel Day is held in Peel, with pit bike road racing, a smaller version of the Monster Energy show, and so on.
Ramsey is kind of in between- more pubs, more people, but nothing like Douglas.
...to be continued...