Mashups are relatively facile to do, very easy to get wrong — and exceedingly hard to go beyond a mere "get right", transcending the apexes and hitting some sort of cross-fusional aspect… like mating a whale shark with a giraffe and having their babies look beautiful. From the rhythmic structure (often 4/4 time with 8-bar chunks) necessitating tempo-matching, to hoping the riff that comes in at one point of Track A accentuates the vocal in Track B, there's a lot to match, and yet… pick an ideal pairing, breed 'em, and you've got magic. It's even better when they have videos!
Here are 2 of my faves:
"Bruised Water" - Natasha Bedingfield vs. Chicane