According to Sergey Galyonkin and Steam Spy, Steam had its best year ever in 2015, adding over 3000 games and generating $3B in sales, $680M of them from 20 games alone.
Galyonkin prefaced his report with the caveat that "the following data was gathered using Steam Spy service. Steam Spy estimates ownership of games on Steam using statistical analysis and is not 100% accurate!"
The Steam Spy stats revealed that while December was obviously the best selling month, thanks to holiday shopping and sales, April was the second biggest selling month, thanks mainly to Grand Theft Auto V making its debut. Rockstar's open world game had monster sales in that month, selling a whopping 2M units for $112M. In general, GTA5 consistently was a huge seller, never being less than the 4th best selling game in any month since its release, even being #2 in December.
Grand Theft Auto V and Fallout 4 dominated sales in 2015 by a wide margin in terms of revenue, grossing $161M and $122M respectively, with the #3 game in terms of revenue being Metal Gear Solid V with $41M. However, indie games matched the top 2 in terms of sales. GTA5 sold 3.8M units on Steam, with Fallout 4 following with 2.7M. #3, Ark: Survival Evolved, followed close behind with almost 2.4M units. Rocket League was next at just over 2.2M. Of course, neither game made nearly as much as GTA5 or Fallout 4 due to them being priced far lower.
To read more about Galyonkin's analysis, check out his Steam Spy blog.