Similar answer and probably cliché, but for me it was the first Dark Souls. I finally played it about 2 years ago after avoiding it for a long time and thinking it wasn't my thing. I thought I hated games that didn't allow animation cancelling because they weren't "responsive". If I hadn't heard so many people insist it's great I would have given up because the character doesn't react to every wild button mash.

Boy was I wrong. Once I understood the combat it was like Zelda (my OG favorite franchise) but better. And brutal. Playing through it subsequently made Elden Ring much easier than it probably would've been otherwise. Exploring every nook and cranny and overleveling helped a bit too I'm sure.

On PC with mods for upgraded resolution and textures (and dsfix) DS1 was a quite good experience. There's still a bit too much BS like hidden paths and even NPCs that are way too obscure, and the game falls apart near the end, but learning to navigate the platforms of Blighttown and besting all the different bosses sharpened my skills like nothing had in ages.