Oh god I think this is the hardest decision we've had to make this entire tournament.
I think I have to go for Unforseen Consequences, because it takes the great level design of Anomalous Materials, as well as the environment that level design represents, and turns it on it's head. I'm a big fan of how the game makes you familiar with the Sector C labs and then has you go through a ruined section of it - it was also great seeing that happen to the trainstation in City 17, and basically every location you visit in Neil Manke's HL mods.
Also, UC is full of iconic setpieces - Waking up in the ruined AMS chamber, the entirely optional falling scientist, the first Vortigaunt (who is deemed important enough by Valve to be referenced in Ep2's G-man Heart-to-heart!), the Akira-inspired funicular ride, the giant canal, the box smashing room...