The most durable would be 2K clear I guess. When applied right, it needs no sanding and polishing in the first place. When you say you have to sand and polish, that's a lot of work though, and 2K is also much more work to mix, apply and clean your tools.
I use Tamiya TS13 gloss clear (or semi gloss or flatt). It's easy to apply, quite hard after it has completely dried, and sands and polishes well. It can attack the base paints though, if too much is used at once.