Dark Souls is up there in my top 3 favourite games of all time, joined by The Witcher 2 and Ocarina of Time. I finished it around maybe 6 times, spent well over 300 hours on it, and still not bored of the game. I didn't get to play the DLC yet, but definitely will in my next playthrough... if only I could actually drop Sm4sh so I could play other games lol. Only playing games on my laptop when I'm bored of playing sm4sh and don't have anything else to do.
The exploration in Dark Souls is just so good. It gives you the handheld Castlevania taste in that regard where it gives you the freedom to go anywhere you want, while all the areas are beautifully linked together. All the boss fights were literally epic and they all had beautiful backstories behind them, some which literally make you cry lmao.
People seem to hate the trial & error aspect of it the most where you try to clear an area to progress, die by something stupid, and then have to do it all again, and describe it as tedious, but I actually enjoy it and feel like its what the game is about: exploration & trial-and-error-based gameplay.