Let's see here, first let's put a few categories and fill them up:
First, I suggest Clash of Clans
which is already suggested in this thread, but it's actually not bad at all once you get into it. then I will suggest you play Plague Inc
Where you basically design/engineer a virus to kill everyone on the planet (very fun to play on small tablets when you are wasting time)
Next up is the legendary Plants vs Zombies
It is funny, quirky, and highly repayable. I can't begin to tell you how many hours of my life have gone to this game. I suggest you truly give it a try.
Next let's talk RPG games:
First, Last Days on Earth Survival
This is still a developing game, but trust me even at this stage it will have you hooked and coming back for more. You basically play as a zombie apocalypse survivor, without taking too much away from the experience let's just say that it is the BEST game in this genre that I have ever seen, you literally have dig deep into forests just to find a deer, hunt it, skin it, and use a tanning bench to harden the leather and use it to make your running shoes slightly better lol.
It is well worth it, if you are a perfectionist who wants to build a gigantic base full of zombie traps and customize pretty much everything I highly suggest you dig your fangs into this game.
Next, Knights of the old republic
I can't really explain this without writing a wall of text, basically it will satisfy your star wars itch FULLY. Give this one a shot.
And lastly Titan Quest
This is basically orgasmic on so many levels, it is Diablo on adroid, and dare I say even BETTER than diablo.
These are the RPG games that I can suggest, and believe me I had to pick 3 out of numerous ones that I played, so these must be really worth it (at least for me).
Now, if you are just looking for innocent 15m riding on the bus gaming time then look into Candy Crush
or maybe even Eufloria
or Osmos HD
This one has a fantasmical soundtrack.
These are just off the top of my head, Let me know if you want more suggestions.