[Nyx]

Relative to the forum I don't think I'm that old so I missed out on a lot of the games previously mentioned, but for my top (nostalgic) favourites I'd have to go with (in no particular order):

-Fallout 3/NV (My first real introduction to open world RPGs, my dad and I spent many many hours exploring the wastelands together as a kid. You can also toss basically all of the Elder Scrolls series in here too as they're very similar and I love those a lot too, but for me Fallout>Elder Scrolls every time. I'd also like to mention NV slightly edges 3 out on quality of the game for me, however since 3 was my introduction to the series it felt unfair to list only New Vegas)
-MW2 (Makes the list purely based on the amount of time I spent playing that at 12. This game was my first real introduction to online gaming, before that I was single player only always. Technically Warhawk for the PS3 was my first ever online game, but I never actually got invested into it)
-Minecraft (Besides Lego (actual physical Lego blocks) this game is probably the main reason I'm taking an Engineering degree not going to lie, I loved this game so much as a kid and I still do and it really helped me build on my creativity as a child, constantly challenging myself to build to coolest house or make the most intricate redstone circuit)
-Pokemon Emerald (I chose Emerald because it's the one I played most as a kid, however they're all generally the same in my eyes so I pretty much love them all equally. Side note, Pokemon basically taught me how to read)

It was really hard making this list, partly because my memory of all the games I played as a child are slowly fading away but also mostly because if I had to sit here and write about all the games that had an impact on me as a kid I would be here for days. For now though, I'd have to say these are definitely the games that had the biggest impact on me overall.