There are so many to choose but if I'm going to pick one of my favorite solo stories
(which could kinda count as a team up as well technically) is "The Supergirl Saga" that introduced the first post-crisis Supergirl, Matrix
for a team-up story
I'm going with Batman/Superman: Public Enemies.
It reads like a silly popcorn buddy cop action flick and that's fine there are still plenty of good character moments for our heroes.
For Elseworlds stories, two of my favorites are little known and IMO very underrated
Superman/Wonder Woman: Whom Gods Destroy
and JLA: Act of God