Hearthstone
Hearthstone is an fun and simple card game. Its free. The strategy involved when you first get the game is minimal, but as you progress through the game it can become more complex. Very balanced (There are cards that are more interesting than others but even the basic cards are fair)
I like it a lot, just a game you can play by yourself to kill some time.
-Currency
Every day you are given a new quest, when you complete a quest (Win 4 games with a specific class for example) you are rewarded with gold. You can spend 100 gold to buy a pack, which allows you to get more cards to play with. You can also spend your gold on the arena. This is where you are given 3 lives and have to create a deck with random cards you select from. The max wins you can get is 12. Once you lose 3 times, you are taken out of arena and given a reward corresponding to the amount of wins you achieved.
-Problems
Sadly there isn't many ways to spend your gold. No trading of cards or gold.

So the rest of the games on this list have nothing to do with currency.

Rocket League
For a £15 game ive already got my hours worth. 900 hours and im expecting a lot more. Its fun, addictive but simple. I like it because it can be taken seriously or played just for fun. Like toribash, there is a huge skill gap. With a little bit of effort you could be pro but its not easy. The game is updated often and there are constant changes that benefit the community (Unlike toribash). Lots of people (Around 120k peak a day) so you will always have someone to play with/against.
-Problems
If you don't have money, that's an obvious concern. When you have played the game for 900 hours, you start to not appreciate good shots or saves so the game can become somewhat... boring? Much better with a controller so uh... that's like another £10 if you don't have one.
Brawlhalla
Do you like Super smash bros? This is pretty similar to it. Free obviously. Can be a great bant with the mates.
-Problems
I lost interest pretty fast. I just play it when my mates want to. Not very fun by yourself.

I have over 60 games and I only have 3 to recommend? Well these are definitely at the top for me right now.
P.S I have preordered Overwatch, the beta was really fun. I didn't recommend it because I don't think its worth it for its price (£30)