Revenholm was like a survival horror game where the controls werent part of the horror. Poison head crabs and superfast zombies up the stress factor a ton, and the crazy reverend guy was pretty entertaining. It was a nice timeout from the constant onslaught of the city17 soldier warfare, and got back to the headcrab/zombie roots of the original Half Life without being redundant. I loved it. Once you are out you get into the more open spaces, the buggy on the highway, and the best part of the game IMO in the assault on Nova Prospect.
Im not a big fan of the citadel levels in either HL2 or EP1, the game play just isnt as varied or fun, but the level design, puzzles, and look are still outstanding. I wont talk about EP2 even though I really wanna, you guys need to get your asses in gear. Ill just say that EP2 is wholly engrossing, and moves the story along a great deal. Its the best game in the package, and that saying a lot.
Portal was a complete surprise, it was a lot more fun than I thought it was going to be. It is genuinely disorienting, the concept is so simple, yet really addicting. I ended up playing it start to finish in one setting after Ep2, it was too fun and funny to put down, and I really wanted to know the connection to HL2. Which ended up being pretty minimal, but not really disappointing given how "worth it" the game itself is. Its written by the same cats that did Psychonauts, which is another truly funny game. The scrawling on the wall, the ending song, what the final boss says to you as you fight it, I was loling far more in Portal than any game Ive played before.
TF2 is just genius, it too is really funny, has a ton of charm and atmosphere, and classes that are easy to pick up and play yet have some depth should you decide to take the time to learn them. Ive found that even on public servers with 16 v 16 teams still somehow manage to come together, sometimes thanks to bitchy people on the mic, but i think mostly because the best way to climb the scoreboard is to play your class the way it was intended. The maps are really well designed, and offer plenty of variety, Granary and well are kinda similar, but other than that the flows and objectives are different enough to keep it interesting.
The custom maps are already rolling out, and Ive played some really good ones.
O, and spies are fucking gay.

ps about portals story and relation to HL
spoiler (click to show/hide)
The objective of EP3 is this thing called 'Borealis", you dont learn much about it, but supposedly its a technology that is made by Apperature.