Among the books labelled as Elseworlds my favourites were the Generations books from John Byrne--including the Batman & Captain America one-shot that kick started the series. After those, my faves would be World's Funnest, the Golden Age, Superboy's Legion, the Liberty Files and Thrillkillers. That's of those I actually bought--at a certain point I realized this was a lot of money (above the price of regular comics) for things that weren't supposed to matter and I became more selective about which ones I would get.
The Imaginary Tales have these beat. They were cheaper (at the time).