Guess if I had to pick 3?
Neverwinter Nights
Smite
Dark Souls: Prepare to Die Edition
[size=small][size=large]Reasons:[/size][/size]
[size=medium]Neverwinter Nights[/size]
I played this game a lot during my childhood and thus it will always have a special place in my heart.
It remains one of my most played games, and for me it has an incredibly high level of replayability.
Plus you can make your own levels for the game or download ones made by other users.
[size=medium]Smite[/size]
I play this game almost religiously. I consider myself pretty good at the game and enjoy it for the mechanics and reaction-based gameplay. I have sunk and inordinate amount of time and money into this game and I don't think I could let it go if I tried.
Super fun if you can get past the plague of garbage randoms as are ever present in any MOBA and find some people to play with.
[size=medium]Dark Souls: Prepare to Die Edition[/size]
This is the game that taught me to have endless patience with anything. It's a wonderful game and I love the intricacies of the combat mechanics. The lack of obvious plot is highly appealing to me as well. I love the way the story is told, and look forward to more games using a similar method of storytelling.
I have played the other two games a fair bit but they don't top this one in my humble opinion.