That's a question my very partner asked me! It's hard, and I think everyone has different terms they prefer. In my case, I love to know how the other person see me, what characteristics they like about me. So I've got my fair share of weird/wonderful nicknames, and they're special because each person made a little story about it, you know?
But, some of the more usual ones are honey, babe, love, sweetheart. I love when people see me as soft/caring, so I quite like those! 💜
Now, talking as someone who has lots of enby friends: ask them! I've seen some people say that asking feels weird or forced, but I think it's a way to show you care about them 💜 I'm glad when people ask me how I'd like to be called, because they're being thoughtful of how I feel, and really, that's a huge green flag to me :D