Are you counting the big "shared universes" as crossovers? If so, I enjoy the Marvel ones, minus AoS. That show tries so hard be "connected," it's just kinda sad now, imo. But I like the way the Marvel movies built their universe for the most part, as well as the Marvel/Netflix ones.
The Buffyverse probably had most of my favorite crossover TV eps. Whenever a Buffy character made it over to Angel, they almost always transformed into a more awesome character.
And I will say one of my favorite episodes of Chuck is the Die Hard crossover ep.
A really random movie one I enjoyed was when they brought in Michael Keaton in Out of Sight, to play the same *****ey agent he played in Jackie Brown (since both were based on Elmore Leonard books). EDIT: dude beat me to it.