Divinity: Original Sin
The newest game on this list, Divinity: Original Sin is the latest entry in the Divinity series. It incorporates a number of classic items in its gameplay, and its combat, environments, and story are all extremely good. It's certainly challenging, but it's also one of those games where you get out of it what you put in. It doesn't hold your hand, meaning that the difficulty can be punishing at times, but if you're up to the challenge, it's a super rewarding adventure.
You can check it out on GOG.com.