My vote was for Air (Galactica)
I haven't been to AT in a while so can't comment on how busy the queues have been, but here's an outsider's perspective on what others have said:
Nemesis is a classic and renowned by fanatics, but when you come to the public it just isn't a total "favourite" because they can find an inverted coaster elsewhere. All aside it still seems to be popular as nobody particularly hates it, so it would still draw in a steady stream. Nemesis is also one of the few rides in the park to truly have a touch of "magic" and creativity to it, so if Alton still want to look like a nicely themed park, keep it. (Plus they have Sub-Terra which, cruddy as it is, cements a theme and they'd have to retheme or remove that ride as well as retheme Nemesis Inferno, a lot of hassle).
Oblivion is well embedded into the ground, for a start, plus the new kid in its area suits the theming rather alright, so it doesn't look out of place even with a flat ride removed from the area. A lot of people I know seem to love Oblivion or at least love to watch it! I have friends who ride it and come away with teddy bears and lots of merch, and I have friends who are too scared but still love to talk about how awesome it looks! Oblivion still generates a lot of talk when you bring up Alton Towers.
TH13TEEN has overcome that hype crash and is now recognised as a great family ride, they reworked an entire area to fit around it and if they removed Rita then that's the only ride left in an entire themed area! It'll stay for a while longer.
The Smiler has not made its money back yet, so it's staying.
Air is a tricky one because contrary to its "meh" reception by the modern enthusiast community, so many people say it's their favourite. Hell, even my partner said that they wouldn't return to Alton Towers if they removed Air, and that even "Galactica" makes them sad. BUT whilst it has a large fan following, it is pretty much avoided by most people who are not infatuated by it, resulting in lower ride numbers. The whole fact that Air is the first SW (on this list) to get a proper re-name and re-promotion is not promising. Though it is not as old as its SW brethren, it has been selected for more than just a fresh paint of coat, because it has grown tired in the fashion of it being boring once you get past the flying aspect. I can't get past the original theme of Air and cannot fathom how out of place "Galactica" will feel. I don't think the "juxtaposition" is as nice as others say. Air also takes up a lot of potential land, and if they replaced it with a new beast of a coaster with the correct theming and maybe another flat, Forbidden Valley could be the perfect themed area.