Legend of Grimrock

This one follows one of the oldest RPG formulas out there - tile-based movement and turn-based combat. You can think of it as a board game played on your PC. Though, unlike most titles in this sub-genre, Legend of Grimrock doesn't pause the action after each move, meaning it doesn't kill the flow as much as one would expect. 

The view is first person, which actually serves this game's crowning feature - secrets! Legend of Grimrock does the secrets exceptionally well. There's hidden switches, floor panels, fake walls, traps and none of it is an obvious character skill challenge (such as it is in most D&D based RPGs). You have to notice the switches yourself, you have to find a way to get past a blade trap etc. Basically, this game made ordinary rocks a must-carry for me.

If you like this, you might also enjoy: any Might & Magic RPG (especially 6, 7 and 8), the Wizardry series, Legend of Grimrock 2

Diablo 2: Lord of Destruction

You must've at least heard about this game, if you haven't already played it. It's a must. I don't care what anyone says about any new attempts or previous attempts to copy it - this game is the king. In case you haven't really heard of it, let me tell you what it's about.

It's a hack and slash RPG, with a top down view, mountains of different monsters with even more mountains of varied treasure. This is all about making your character become as awesome as possible. There are 30 skills per character, each unique and fun in their own way. There is so much different gear, with different grades and qualities that I dare not go into it. The amount of playing hours it offers is ridiculous and it's very fast-paced to boot. You'll be clicking and killing a lot! It's not uncommon to see a group of 20 monsters collapse with a single blow of a skill-charged hammer attack.

Diablo 2 perfected this formula, especially with its expansion pack. There have been many attempts since to replicate it, but no game has managed to capture the charm, atmosphere, character diversity and originality, or the huge (yet meaningful) amount of item and monster variety. The one that comes closest would be the first Sacred game, which I could also wholly recommend to represent this category.

If you like this, you might also enjoy: Sacred, Sacred 2, Diablo, Torchlight, Torchlight 2, Titan Quest, Dungeon Siege