1. Cave Story

2. Portal

3. Descent

4. Dark Forces

5. Marathon

These are the games that spark the most nostalgia from me when mentioned. I don't care if the graphics sucked out loud on some of them, IMO they really define the genre they've been placed in.

Nothing can beat Cave Story. Ever.

Quote and Curly Brace: <3

Misery: <3

Mimigas: <3<3<3

Portal was just so unique I couldn't help but love it.

Companion Cube: <3

Descent was the first game to have truly stunning six degrees of movement, and the first one I played, at that.

Everything about it owned.

Dark Forces and Marathon were just plain fun. I think they were the first FPS I ever played, and both gave me an honest fear of deep, dark water and being crushed.