Standard price: $29.99
Genre: 4X turn-based strategy
Steam Store link
It's hard to say this one is a "hidden gem" considering how often it goes on sale, but when Civilization V is still the top 4X game on Steam, I can't help but bring this baby up in this list. It's fantastic.
Endless Legend is exactly the type of evolution I, as a regular 4X strategy player, would like to see in ground-based entries (as opposed to space titles like Sins of a Solar Empire) to the genre. Instead of taking pages from popular staple Civilization V, developer Amplitude Studios went in their own direction and created a basically triple-A quality 4X with more of a focus on unit management over city management.
The game's Hero system, which has players customize Hero units' skills and equipment to better lead their squads to battle or oversee their cities, is fresh, fun, and adds an additional layer of strategy,
The gameplay variety between factions (and your ability to customize your own faction) means you can play pretty much the way you want -- within reason.
If you like 4X games, there's pretty much no reason to not pick up Endless Legend. Except maybe the $29.99 price tag with additional paid content DLC. But you expect to put that sort of investment in a high quality strategy game.