Doom and Doom II.

They certainly were not the first games I played. For my young self, games before then were either trivial games which you can figure out and play easily or difficult games without manuals which held my interest for brief periods of time. Games were (and are?) a certain difficulty and operate as they were designed. For Doom and Doom II, that was different.

Doom and Doom II were the first games I used cheat codes in (because they were the first games that I knew cheat codes for). The cheat codes in those games spoiled because they did more than just "make you invincible" but they also let you walk through the walls of the levels (noclip). They allowed you to see how the game worked (at least in a small way). You could also level jump (a more common cheat code) so that you can see levels that I did not have skills to reach. This made the games more than just a triviality since I could keep exploring and trying new things despite my skill level.

Those games were able to be modded though. You could easily get CDs with plenty of mods that changed the weapons, added levels, completely changed the game, and so on. This was the first game that I ever played that could do that. The CDs also came with editors which let me dabble in messing with weapons myself (where I managed to get around 1 FPS with all the rockets I fired at once from a rocket launcher). As such, the games could be made fresh and new again by modding it to be something different.

Those games also had a great sound track. It seems like a minor thing (and other games have great sound tracks as well) but I learned that music significantly influences my like or dislike for a game. Games that I played before didn't have bad music per say but nothing earlier really grabbed my attention like Doom and Doom II.

I do enjoy many modern games. Still, I miss that games typically do not have cheat codes (and things like noclip are a rarity in any new games) and modding has never seemed as "wild" as some of the Doom mods that were created back then. If Doom was never around, I'm sure that some other game would have grabbed my interest in different ways (likely it would still have a great sound track though). However, I would have likely missed the wonder of seeing how a game worked and seeing a game be modified.

Fortunately, these games are still playable today and still have new mods released for them today. As such, I can take a nostalgic trip and play them whenever I want.