Personal opinion: the merge games are the most efficient and effective earning games
So far have tried about 15 games in total, ranging from all types of long and short form games, like Disney Solitaire, Resident Evil SU, those cat games, etc, and I personally think the merge games are the easiest to earn from. No money spent btw. I have so far earned a total of £45 on just three merge games alone.
I personally think they are the easiest to do and also pretty sustainable. Some of them are pretty money hungry but still doable as f2p. Highly suggest Cook & Merge or Tasty Travels.
My absolute “do not even bother” games are games that look so promising with high fast rewards like Disney Solitaire lol. They are designed for you to fail unless you spend.