Ego showed up at the end of episode 2 and in that timeline Peter Quill would have probably chosen to cooperate with him fulfilling the Expansion. So maybe Ultron would encounter Ego.

Peter was dead at that time because Ultron killed the guardians, but he already fought a celestial which was ego

Ultron and his bots blew Ego up in What If…? episode 8 so yeah. If we’re talking comics, idk since Ego isn’t a celestial in the comics.

I think it would be a tie if Ultron (with infinity stones) fought against all celestials (not including Ego)