My favorite(s) depend on which fandom - for L&C it's probably Lois & Cl - Kal-El

For U.F.O. it's A Dog's Day - Straker is the same one as in Lois & Cl - Kal-El and the other Planes series stories so I personally consider it part of the Planes series but it isn't a crossover.

And for the Superman Movieverse: Happy Hour at Dulin's Bar (Which is part of the Shadows in Darkness series)

Big Apricot Superman Movieverse
The World of Lois & Clark
Richard White to Lois Lane: Lois, Superman is afraid of you. What chance has Clark Kent got? - After the Storm