A while back I made this choice and chose Molly since normals are too normal for me so I didn't want more than one and I was going to get Sally.
My current lineup is Apollo, Diana, Kid Cat, Elmer, Agent S, Marshal, Sally, and Static.
Sherb and Jeremiah aren't permanent, but I'm getting attached to Jeremiah

and he might stay, but I won't count them for this.
I got most of my villagers out of nostalgia with Elmer and Marshal being new. For the ones I only have 1 personality of, they're the ones I like the most for that personality. Agent S is the exception because I like a lot of the peppys, but her and Kid Cat are paired, so Agent S is locked in. So it comes down to Apollo and Static. I'm more sentimental about Apollo, so I think I would keep him over Static. However, they both have the music hobby and I like to think they have jam sessions together, so letting him go would be sad but I think the only choice.
I'm clinging onto the small hope than Nintendo will give us more villager plots so I can have Ruby as a permanent. I had her for a bit and then gave her to a friend. I did the same to Julian and I'd love to have him back.