Sequels always suck. Seriously, when have you ever played a sequel and it was better than the previous one? Aside maybe from Mass Effect. And possibly Half-Life. Or Civilization. Or Arkham City. Or Doom. ...OK, maybe sometimes sequels work out fine, never mind.

That said: if you've never played Portal, I highly recommend to play that first, because Portal 2 totally builds on the story in the original. While not absolutely required, playing them in reverse order would be weird. If you don't have the original, get it here. Or enter for the bundle.