For me there are two ideal components.
- A compelling setting, either a world I want to explore or a genre that I can get excited about. Right now when I am not playing RM I am dabbling with Ghost Ops. In actual play it is super light and fast to run but is miles apart from RM in just about every way.
- The absolute minimum of rules. I have a brain, common sense and imagination. I can fill in the blanks once I know the flavour of the game. I know many GMs and even players dislike it when things come down to arbitrary GM decisions but I quite like that. I would never simply decide that an attack hits or misses but on the other hand there are times when I don’t want the characters to drown just because the rules say they do, I want them washed up on strange foreign shores.
So I think my perfect game is one that absolutely demands to be played so much that it is all you can think about when I should be working on other stuff but has a rulebook I can memorise and run without referring to. That may be a tall order but I have played a few of them. To be honest even RM fits into that definition if you have a setting you love. All the heavy lifting is done during character creation, resolving most skill checks and role play situations require no rules. It is only combat that chains you to tables. (I am assuming here that each player has their spell lists printed out as part of their character record)