For me it's Chrono Trigger. I always want to play it. I want to show it to my children. I hope it will be regarded as a masterpiece for generations to come.
Yeah, the graphics are surely dated, in my opinion it still looks nice, HL has a great art-style. The gameplay is still great, smooth and fun, even if it's not like modern games with dozens of mechanics, this style of FPS isn't outdated.
I never played the original one because the graphics are such a turn off. I'm considering buying the black mesa fan project though. Graphics are from the Source Engine (HL2 essentially) which are way a lot better, even if slightly outdated today.
I started playing Valve games with HL2, and didn't play HL1 until the Black Mesa game came out. I highly recommend. It's a fantastic game and the story made bolder choices than the new games. It really gives a sense of wonder in parts, especially on the back half of the story.