1. Crusader Kings 2 (In terms of Grand Strategy this simply offer the best experience I have ever had. It offer the widest variety of tactics to use to expand and play, plus I just can't get enough of it. It is the game I have been the most addicted to in the past 15 years. And this is not counting mods like Elder Kings of AGOT which are complete and awesome overhauls.)
2. Gothic 1, 2 & 3 (The series that made me love gaming, the first game I played was Gothic 1. And for that point in time, it simply dragged me in and made fall in love with the character, the setting, the way it handled things. Ever since these games I still can't play and finish linear games. Gothic spoiled me with its branching story mode. My heart literally broke when Arcania: A Gothic Tale came out, shame as it killed the series for me.)
3. The Witcher 1 & 2 (These games brought me so close to the Gothic Series. While different story, mechanics, everything, the feeling was the same. It made me enjoy RPGs once more, and Witcher 2 was absolutely beautiful both in visuals but even more in story.)
4. Neverwinter Nights (The Castle Management part was simply extremely fun, wish it had more of that. With more games had that feature done in similar and more expansive way. Dragon Age 1 DLC did it in the same way, but more limited, with less features.)
5. CK1, Vicky 1&2, EU 1,2,3 & Rome (Well, I honestly didn't like them as much as I did the previous games on this list, and all of them combined have not occupied my time like CK2 did.)