Odd as it may seem, get two, not one. They’ll entertain each other, and be a lot less bored than one cat alone. Get an already bonded pair if possible.
The alternative might be an older cat who’s used to being alone and seems to prefer it. Well taken care of cats usually live into their late teens so a five to ten year old cat has a lot of life left. Some of them do still need a lot of space, others don’t, see below.
Get cats who have never been outside, and if possible who have never had a large house. They’re a lot less likely to miss what they’ve never known.
Some high energy young cats (especially males) will drive themselves and you crazy in a small space; other cats will be fine with it. I’d go for cats old enough so you can judge this part of their character.
Cats use vertical space. Give them plenty of things they can climb on, and they’ll have a lot more usable space in that efficiency than you will.