My favorite RPGs are:
-Morrowind, it has amazing story, big world to discover, you have a lot of freedom, and the atmosphere is great
-Diablo, it's a great RPG. I'm a fan of dark fantasy games, so I like this game very much. It's creepy, it has great replay value, and the story is great. Not a fan of Hellfire DLC tho.
-Daggerfall is a great game. I have never completed the main quest, but the game is great. There is a lot of great mechanics, and dungeons are great
-Baldur's Gate. I have never gotten really into it, but I enjoyed it very much
My favorite sandbox games are:
-Skyrim. It has many flaws and it's far from a "best RPG game", the game isn't even an RPG. But I still enjoyed it
-Mount and Blade Warband. It's a great sandbox. It gives you a lot of freedom, and it's world is great. It is one of the best games I ever played. It has no storyline, but who cares when the game let's you make your own.
-I love GTA titles, especially San Andreas
-Total War games are great
-I love Age of Empires games
-Reign: Conflict of Nations is a great game that sadly is largely unknown
RuneScape, been playing for almost 13 years.
The Commandos series, awesome strategy game. Destinationaris is a huge mod with insane difficulty, if you like that.
Anything Paradox, specially Crusader Kings II and Europa Universalis IV.
Papers, Please! and Return of the Obra Dinn were awesome.
Age of Empires II and Rise of Nations for RTS.
I also think of the Half-Life series, Antichamber, Medal of Honor AA, GTA II, III and San Andreas, Portal, Return to Castle Wolfenstein, Terraria, Minecraft and Undertale.