39. JLA vs. Avengers
The basic concept behind the beginning of JLA vs. Avengers is that Krona and the Elder of the Universe known as Grandmaster decide to have a little competition, and as part of their competition, the Justice League and the Avengers have to compete to collect a certain amount of items (all notable items from Marvel and DC history, stuff like the Spear of Destiny and the Ultimate Nullifier). Since both teams are after the items, they have to split up and they have conflicts over who wins each item.
Each match-up is handled very well by writer Kurt Busiek and artist George Perez, but none match the coolness of Batman vs. Captain America, where the two spar for a little bit before each man realizes that they are too evenly match, and decide instead to work together and break the news to their respective teams that they are all being played for patsies.