This is such a cheat, so in fairness to the other games on this list I couldn't just make this game number one, but the quality is there in The Orange Box for it to easily be number one on this list. I'm going to break the games apart and say a little about each one of them separately, to give each one a little time to shine.
Half-Life 2 - So The Orange Box was my first run in with this game and while it had aged *slightly* by the time I got around to playing it in terms of design, it held up better than almost every other FPS from the previous generation (Halo being the only exception I could think of) and became one of my favorite games while playing through the game for my first play through. I do think the gun play is slightly weak, in comparison to games like the Halo series, but the shooting feels more satisfying than say, the BioShock series.
So many amazing moments in the game though: arriving and escaping City 17, Ravenholm, The Coast and bridge (had huge vertigo crossing the bottom of the bridge and those cursed barnacles (ughhhhh) didn't help matters any!), controlling the antlions and finally, getting to use the most satisfying gun in gaming that's not named the BFG9000, the Gravity Gun just led to some of the most amazing moments in gaming.
Half-Life 2: Episode One - Probably the one game from The Orange Box that I have the least to say anything about, probably the biggest positive I have to say about it is Alyx Vance. They perfected her A.I. in this Episode and to date, I don't think a computer controlled A.I. plays better than Alyx does. Oh and the trick she pulls on you in the tunnel, that got me so good!
Half-Life 2: Episode Two - I don't care how often or strung along we gamers are, but Episode Two is the reason why I still want and yearn for an Episode 3/Half-Life 3. As good as the vanilla Half-Life 2 is, Episode Two is even better. Alyx is an even better companion here, the gun play is really, really good for the first time in the series. Again, so many great moments: Alyx getting wounded (don't think I've ever been so shocked by a game before) and teaming with the vortigaunt to save her, getting into the antlion nest, defending the vortigaunt's while they heal Alyx (which may just be my favorite game play moment this generation!), "prepare for unforeseen consequences", running into an Advisor, arriving at White Forrest, D0g fighting a Strider (!) and the attack on White Forrest which again is another moment where the gun play really shines and then finally, that ending. Just please Valve, give us gamers something, some hint that Half-Life 3 is on the way, please please please!
Team Fortress 2 - So, caveat, but as with most games on this list, I played this on a console (360 in particular) so I'm maybe not the best person to talk about this game, but I can say that I probably played it for about a year straight and enjoyed my time with it thoroughly. I love that death match doesn't really exist and it forces you to play a team game, which I mean, the word team is in the title, so that should give you some kind of clue as to what you're in for here. All of the classes were so much fun, though my favorite was probably the Spy. Just trying to convince the other team that you were on their side while leading them to a trap or ambush from your team is just genius multiplayer design. But yeah, Team Fortress 2 is one of my Top 5 favorite multiplayer games this generation and again, seeing as how I played it on consoles should say something. I want to check out the PC version, but it just sounds unrecognizable to what I've played (hats!?) and given the skill level of the players that are there now I'm sure, I'm pretty unlikely to do so.
Portal - Probably the most charming game on this list and I don't know that it created a genre, but it certainly popularized a genre, First Person Puzzle Shooter, we'll call them. Just an absolutely, amazing game that has an objective and superbly accomplishes that objective without overstaying it's welcome. GlaDOS is one of the best characters of this generation, as well as the companion cube is and the turrets are pretty great too. I love the hints that Portal is somehow tied into the Half-Life universe, but yeah, Portal is just an amazing, small game than anyone remotely interested in playing a video game should try out. It's so accessible and is one of the funniest games out there. So if for some reason you have yet to try Portal, definitely check it out!