Here's a quote of my Portal review, where I explain why you should play that game before Portal 2:

Originally posted by "me, in my Portal review":

Play this before Portal 2 if you can.

The story between Chell and GLaDOS presented in this game is such a huge part of the motives in Portal 2. Do yourself a favor and boost your Portal 2 experience to the maximum by playing this one first.

Returning to this after having already played Portal 2 may not be as great and that's a crying shame, because this is a wonderful game and the perfect introduction to the Portal universe.

This is a great game! The story, the puzzles, the music, the atmosphere, this game delivers on all fronts! The co-op is also awesome and after finishing the multiplayer campaign there's still huge amounts of custom levels in the Steam Workshop.

If you have a friend who is about as good with the mechanics of the game as you are, and you're both quite competitive, the Gelocity racing levels are a must try! There's shortcuts and also traps you can activate to hinder your opponent's race. There's three different tracks (1-3) and they're all great. I think I had the best fun on track 2, go try it!

In case you wondered, I'm not affiliated with the maker of the Gelocity tracks in any way. :P