Maybe I've got a bit of bias, since I grew up playing this game, but it's so distinct from anything else in the series—that makes it special to me.

There's this serenity to it. The NPCs you meet can only be interacted with so much before you have to leave them behind and move on with the game, and for that reason it's an isolating experience. But that isolation isn't lonely or painful. You're a vault hunter, little more than a mercenary, and the planet you live on simply isn't the place for a social simulator. Really, you spend most of your time doing quests, traveling the vast borderlands, collecting loot, and shooting baddies. Alongside an awesome, ambient soundtrack in the background, this game establishes a strong atmosphere for Pandora and sets a tone for the rest of the games. While, technically, it's far from the highest quality game in its series, I think it's essential to understand just what Borderlands is. For that reason, it's special!