I'm definitely partially biased towards Chrysto and Octocube Games, but the idle/incremental games that he/they have released are absolutely phenomenal.

If you enjoy games that have certain loops, with parts of the game throwing in a new set of stuff to figure out, you'll absolutely love ISEPS. I played for over a year before losing access to my savefile and whatnot, but I can tell you without a shadow of a doubt that it IS possible to play completely F2P as there are plentiful premium currency options... but it does require watching ads, if I'm not mistaken. Hence, for a ridiculously small price to have ad free, this is certainly a wonderful game to play and it will keep you entertained for a long, long time.