For me it would have to be Riding Shotgun; I think on the whole it's the most consistent, well-paced and wonderful chapter in the series so far, up there with greats such as Undue Alarm, Nova Prospekt and Follow Freeman. It's not overly long and hasn't a single, unwanted set piece or tedious bit of gameplay. The introduction of the Hunters at the Radio Tower is superb, and works well as a gameplay scenario, and along the same lines is the Chopper showdown in the freight yard, which is highly entertaining and rewarding. Stand out moment would be the Advisor Barn, because it's just a bit of brilliance that Valve need to be applauded for. Abandoned, derelict barn in the middle of nowhere; a malevolent, highly capable alien intelligence takes up a corner in an an effect not too dissimilar to a spider. The contrast is brilliant, eerie and awesome.