Chess uses the ELO-rating system (and many other games). It works really well in PvP games with matchmaking - the rating is accurate in reflecting the players relative skill compared to other players. As I've stated in OP though, the ELO-rating system is not really what I'm looking for.
I didn't mean to say "use whatever Chess uses".
I meant to say "there are probably many discussions about Chess rating systems over the years. it might be useful to see what have been discussed, what alternatives to the ELO-rating systems have been proposed, and why they have stuck to ELO".
Edit: Oops, sorry, I didn't realize it was for a singleplayer game lol.