I’m a sucker for things that are blue. I throughly enjoyed “Direct Intervention” from Episode 1. The interior spaces and the captivating citadel core was really quite amazing. Although it wasn’t very challenging; I simply loved the the change in theme from what is stereotypically combine.
Half-Life: Haven’t played it very much, but I love the Resonance Cascade sequence, at the end of Anomalous Materials.
Half-Life 2: The whole “Our Benefactors” level. The two pod rides are excellent.
Half-Life 2 Episode One: The thrilling ride in “Undue Alarm”, the “Direct Intervention” level and the exploding Citadel at the very end of the game, when Freeman is on the train.
Half-Life 2 Episode Two: The Portal Storm at the beginning and the Strider battle at the end.