TV crossover

What's your favorite crossover episode from TV?

What is one you would love to see made?

I'm not sure if it's a favourite, but I used to really love the Jimmy(from Jimmy Neutron) and Timmy(from Fairly Odd Parents) power hour special from Nickolodeon! There was also the Grim Adventures of Billy and Mandy x Codename Kids Next Door, Generator Rex w/ Ben 10, and The Jetsons Meet the Flintstones! Those were all pretty cool.

I would love to see some sort of Steven Universe x Star vs. forces of evil special.

I remember when Lilo And Stitch: The Series crossed over with Kim Possible, as well as the Jimmy/Timmy Power Hour special (Jimmy Neutron x Fairly Odd Parents crossover)
Not sure what two series I'd like to see crossover, though.

for me it's gotta be when Brooklyn 99 and New Girl crossed. Two of my absolute favorite shows. I was hyped!