Omg anon this is such a brilliant and hard question I've been thinking about it all night. I dont think I can narrow it down to one each, but here's my top picks:

Ateez should cover:
1) Savage (it just matches their vibes so well, and they would crush the dance! I think it would look really cool with 8 people).
2) same with slow dive... I know demon line would be too much with that one. Slow dive is sexy asf and I feel like ateez is on that beat lately
3) self-indulgently, Atlantis. I love the vocals and I think it would be really fun to hear Ateez try that out
Runners-up: Favourite Boys, Golden Goose, Black & Blue!!!

A.C.E should cover:
1) also self-indulgently.. turbulence. Such a gorgeous song with raw, emotional vocals, and that is one of the things I love most about a.c.e. they would crush turbulence and leave me depressed for the rest of my life I fear. I would ascend to the heavens
2) NASA !!! MAYBE I am just on a nasa kick but I feel like this one is the tempo and style that would reaaally suit a.c.e especially the rap line
3) SILVER LIGHT IS SO A.C.E CODED CHANGE MY MIIIND
4) had to add a 4th bc... Take Me Home.
Runners-up: Masterpiece, Lemon Drop, Siren, Eternal Sunshine .. there's so many but ill end it there lol lmk if you agree with my picks !!