There are a few that work really well for me.

DJ Topcat - 50 ways to kill your lover

Paul Simon - 50 ways to leave your lover vs. Morcheeba ft. Slick Rick - Women lose weight

Excellently matching contents, makes this a really good fit.

https://bootiemashup.com/top-ten-albums/bootie-top-10-august-2006/dj-topcat-_-50-ways-to-kill-your-lover-slick-rick-vs-paul-simon/

https://www.youtube.com/watch?v=2DZqUapC5tQ

Also really good:

DJ Schmolli - Wicked Wedding

Chris Isaak - Wicked Game vs. Billy Idol - White wedding

https://www.youtube.com/watch?v=jWsExznfT-E

And a third one which feels like it could have been an original song.

Loo & Placido - Toop Toop Groove

Madonna - Into the groove vs. Cassius - Toop Toop

https://www.youtube.com/watch?v=dWXDIDNiUHo

Play that Funky Music, Rammstein is pretty fun: https://m.youtube.com/watch?v=tcUB-3lud60&pp=ygUfcGxheSB0aGF0IGZ1bmt5IG11c2ljIHJhbW1zdGVpbg%3D%3D

I really like "All I Want For Christmas is the Beautiful People (Mariah Carey and Marilyn Manson)". I always like to share with people who complain about one of the songs during Christmas time, as they usually do enjoy this one!

https://youtu.be/A1X3d2zWx94

I assume the PipedBot will generate a link, otherwise I'll edit it in later

Amerigo Gazaway - Fela Soul - Fela Kuti vs. De La Soul is an album I still listen to regularly. Feel Good Inc. is a stand out.

All 3 Beastles albums by dj BC are really well done: dj BC presents The Beastles, Let It Beast, and Ill Submarine

Sweet Jane (With Affection) by Two Nice Girls isn't a mashup, per se, but a blending of Velvet Underground's "Sweet Jane" with Joan Armatrading's "Love and Affection."

Metallica Goes to Punjab, Maiden Goes to Hollywood, Rolling Confusion, And Krishna For All. More but i kinda get lost in listening to them and stop typing...

Here is an alternative Piped link(s):

This Green Day/Collective Soul mashup

this Taylor Swift/NIN mashup

this The Wiggles/Slipknot mashup

Piped is a privacy-respecting open-source alternative frontend to YouTube.

I'm open-source; check me out at GitHub.

All Star with The Sickness is, for me, the perfect mashup, to the point where I can’t separate the songs now.

Honorable mention to any of the many Thomas the Tank Engine mashups (like this one, for example).

I've not actually heard any in so so long, but Bad vs Ghostbusters was a classic mashup back in the day (with sound quality to match) lol

I’m going to listen to all these tomorrow

Gotta be the old faithful "Poolside Identity" by nakinyko. It's a Chuunibyou soundtrack x Kendrick Lamar, certified trash.

For a pop mash, I'll throw in "I Wanna Be Alright" by Sim Gretina, a mix of Ariana Grande & Whitney Houston.

Or an absolutely derelict meme combo of Valentino Khan & Dirty Rush by Pluffapuff (Ali-A's intro music and Mafia City ad music).

Little Sickness. Disturbed + Of monsters and men.

https://music.youtube.com/playlist?list=PLnTDi-o-XsAvlIPFFFVQVdq7orzzzVeg8&si=IsEHM8quWUuFEN9i

I've connected many of my favorites on these playlists.

https://music.youtube.com/playlist?list=PLnTDi-o-XsAu3nUsjCgrTHYibgb6h9KRq&si=AjwcleB8iE77wp0v

Grizzly Bear v The Knife is one I come back to.

Dean Gray - American Edit