Thief. The first three, at least. (*)

Splinter Cell as well.

Basically, Splinter Cell got me into what became my favorite genre: stealth.

Then I went and played Thief because stealth, and just everything about those games furthered my love for not only stealth, but it was my gateway to immersive sims as well.

Also, the level "Robbing the Cradle" in Deadly Shadows will always stick with me. As someone who hates survival horror, that level was both fucking incredible and also a nightmare that I had to use a walkthrough to get through quickly.

KOTOR got me into RPGs, and New Vegas came around and has stuck with me ever since.

And Skyrim I guess, since I've played it way too many times.

Should probably add Prey (2017) as well. Incredible game. For that matter, Dishonored. Both have a permanent place in my mind. And when I say Dishonored, I mean the first two, although I also enjoyed Death of the Outsider.

(*) I actually enjoyed Thief 2014 for what it was. Not nearly on the level as the originals, but it was still okay, if a sadly forgettable entry which kind of put the nail in the coffin for future releases.