Serious answer for 1v1 only:

1) Rimuru (WN end of series) may win AFTER Ainz used up everything including world items, RMT items, all the wishes, and all the different skills and magic.
Ainz can still win if he can wipe out all of the Rimuru existences across multiverses at the same exact time including any other isekai series Rimuru has made guest appearances in.

Otherwise WN EoS Rimuru can pop right back into the scene after the last one was killed and harassing Ainz to no end until Ainz gives up and surrender. He probably will not kill Ainz as long as there is zero permanent casualty to Rimuru’s friends and followers.

2) Ainz will absolutely win against the latest LN version of Rimuru. Just do a time stop with enough setups to completely destroy Rimuru, or just write him out of existence. Maybe death magic also works on this version of Rimuru.

3) Ainz can win if he can time travel to a point before Rimuru (any version) naming Ciel.

Wholesome answer: They will visit the isekai restaurant to discuss about their followers and give each other a big hug. Ciel can help Ainz to reprogram all his followers to his likings. Then Rimuru will travel to all the other isekai to bring everyone to the restaurant if they cannot do so themselves. After the party Rimuru can help all those who wanted to go back to their original world, and the ones who don’t are welcomed at Tempest if they planning to visit.