I'm going to probably sound really dumb by asking this..but what's an idle game? Do you mean indie?
Anyway, I don't really play idle games anymore, but when I did, AdVenture Capitalist and Clicker Heroes were the games I put the most time into.
Clickpocalypse 2 (also known as minmaxia) is a great idle rpg, also Candy box 1 and 2 (although they're dead now :( )
My favorite is Candy Box, but the first took me a long time to finish because it didn't have a real save feature. I would leave it on while I was at school, hoping the browser would still be on when I came back.
You can still play through the links he left here
https://candybox2.wordpress.com/