There are hundreds of games over the years that I have thought were awesome and sank alot of time into but below are the ones that I find myself re-installing or finding the disk/cartrage for over and over.
Final Fantasy 4 SNES version (FF2 in the US): first RPG I ever played.
Mass Effect: entire trilogy awesome story, great characters, lots of re-playability.
Skyrim: took what I loved about oblivion and made in 10x more awesome.
Fallout 3 and New Vegas: took what I loved about oblivion and added guns, mutants, ghouls, and a post apocalyptic earth.
Castlevania SOTN: Timeless gameplay and hidden endings. I completely agree about Diablo II, it's a top game, I play it over and over again. It's a game that grabs you from the first minute and doesn't let go. I also often return to projects like Mass Effect or Skyrim, because they offer not only an amazing plot, but also a huge world that you want to dive into again and again. It reminds me of the gambling games I read about on CasinosFest: when you already know how to play, but every time you experience a new excitement, as if for the first time (especially when new platforms with Google Pay appear). The thing is that these games give many opportunities for repeated games with different results. That's their thrill. For me, Red Dead Redemption 2 is also like that. Father of the Metrovania genre need I say more.
Diablo II: this game ate a few years of my life lol.
I completely agree about Diablo II, it's a top game, I play it over and over again. It's a game that grabs you from the first minute and doesn't let go.
I dunno about favorite but I have just discovered a game called I Am Bread.
Challenging...