I didn't go out of my way to read Judge Dredd's separate encounters with either The Aliens or The Predators, but I couldn't pass up the crossover of all three (four, since scripter John Layman works Judge Anderson in there as well).
It's a solid action-story that gives all the heroes and monsters a chance to shine, but nothing more.
Given that PEARLS BEFORE SWINE has done much to keep the spirit of crossovers alive in comic strips, I should cite at least one example, with the always spoofable CATHY.
From writer Alan Grant and artist Martin Emond (who regrettably took his own life in 2004) came one of the few nineties comics I frequently enjoy re-reading: a one-shot LOBO/DEADMAN teamup, whose inside title really is "The Brave and the Bald." (Technically the ghostly Deadman isn't bald; his spectral self reproduces everything about his former mortal body, including a skin-tight mask covering his face and a full head of hair.) It's a nice contrast between Lobo's untrammeled nastiness and Deadman's relative goodguy-innocence.
Here's one of the earliest comic-strip crossovers I've come across: in this abortive 1922 wedding for Winnie Winkle, her wedding party is attended by other luminaries who shared the same publishing syndicate. I don't know all of them but can place Walt Wallet and Skeezix from GASOLINE ALLEY, THE GUMPS, and HAROLD TEEN and girlfriend (the latter on far right).
The last issue of ACQ's UNKNOWN WORLDS title appeared with this striking "rage against the machine" cover.
A few generations later, some reservation Indians are being menaced by greedy white men, so Mandarra's spirit is released from inside a tree that's implicitly been standing for a hundred years without ending up in someone's fireplace. While there's no need for the black-outline Great Spirit to appear in the latter part of the story, for some reason either artist or writer decided to stick him in there, for whatever dopey reason.
Manga serials are not known for crossing over with other manga serials maintained by other creators, but here's a couple where it's happened recently.
NISEKOI did a two-part crossover with a less well known manga, OREMONOGATARI X. pretty forgettable.
A more lively crossover is one in which the popular NAGATORO literally collides with another lesser known, TEJINA SEMPAI.
Here's yet another largely unexplained convocation of the "Horrific Trio" in an episode of SPIDER MAN AND HIS AMAZING FRIENDS. In it Dracula comes to America, hypnotizes Firestar and takes her back to Transylvania, where he's served by a wolfman and a sort of "Frankenstein robot." Iceman and Spider-Man follow to rescue the maiden from the monsters.
