If you're looking for a completely fresh FPS experience, you will find it in the Orange Box.
As for Portal, it has some of the most wittiest dialogue a video game has ever seen. You will constantly find yourself chuckling at GlaDOS's dry, sadistic lines. The portal gun makes for some brain-twisting(for lack of a better word) puzzles, and, surprisingly, the game has a very intriguing storyline that will pique many questions about the protagonist's role in Aperture Science.
Finally, Team Fortress 2, as the multiplayer portion of The Orange Box, will be a completely fresh experience for COD4/Halo/R6V veterans. You will find the 9 classes as distinct in both their visual design and gameplay aspects. Each classes cancels out other classes, making for an extremely balanced game. This means there will be always be a great mixture of classes represented in each game, and that most matches will be virtual tug-o-wars. TF2, perhaps better than any other game, encourage teamwork for success. If you're looking for a team-oriented, unique multiplayer experience, you will find it in TF2.
Here is the final verdict