Crusader Kings II is utterly magnificent. It's still by far the best Grand Strategy game that Paradox have ever made, a brutal cutthroat game of thrones, bloodlines, fragile alliances and real people rather than numbers to crunch.
But don't take my word for it. It's free to play on Steam this weekend.
If you want to buy it, however, pick it up in the Paradox Grand Strategy Collection, which also includes three other games for £6.90! I assume there must be a misprice somewhere, because it's cheaper than the solus game itself!