I enjoyed VINELAND a lot and think it's one of his better ones -- though only read it once so far, in fact I think I will get it out and give it a second read now. "Inherent Vice" is a bit similar to that one (hippies and detectives are involved.)

The hot air balloon one was "Against The Day" -- took me nearly a year to get through, in the end I thought it was just a shaggy dog story (I don' geddit.) Probably more like "Gravity's Rainbow" than anything else he has written, but I may be a bit unusual in not being much of a fan of either of these!

However I loved "Mason & Dixon" which is just as long and rambling, and "even worse" is written in 18th century dialect. But worth the effort if you have several months to devote to an epic! I think it's his "most masterpiecish" (so far.)