Once again players assume the role of Gordon Freeman, a scientist destined to save the Earth from hordes of gruesome alien attackers. Half-Life 2 takes computer games to a new level of realism with incredible in-game graphics that recreate familiar-looking city streets, and blend them with strange and grotesque monsters, resulting in a believable and at times unsettling environment. Objects and characters feature accurate physics properties, behaving with the appropriate mass, density, and physical properties of their real-life counterparts. Extremely intelligent computer AI result in enemies that plan, adapt, and respond to the player's actions.

That sums it up there. Anyway play HL2 followed by the episodes. Then feel free to play Portal and Team Fortress 2 whenever. They don't really have much to do with HL2 as of yet.

quote Game Hawk

I've never played the original Half-Life either, but Portal goes first because it's easily the best of them all! Then Half-Life 2, then Episode 1, then Episode 2. Portal never even refers to any of the Half-Life's.

Er, not sure what you qualify as referring to Half Life. There's clearly a powerpoint about Black Mesa in Portal and a subtle nod to the Combine by GLaDOS. No one goes out and says 'Gosh that Gordon Freeman is a swell guy' or anything, but it's obviously taking place in the Half Life universe. As to what point in the Half Life series, we're not sure.

Portal -> HL2 -> HL2 ¹ -> HL2 ²

But I'd play HL2 before Portal just because it's more of a story [in fact Portal is really only like a bunch of tests].