Episode 2 has been the high point of the series for me so far. Episode one felt a little bland, with too little story to push it forward. Once you got out of the Citadel at the start, the rest of the game was one long slog to get out of the city, just split up into several different sections of city. Contrast this to Ep:2 where you've got Alex's near-death, delving into the antlion nest, trying to get the car, the copter chase, do I need to go on. You never loose sight of the overall objective, but you always have a smaller objective in sight to guide you along.
On top of this, the combat and enemies are more interesting as well. The hunters are a great new threat and very fun to fight, the big fight setpieces are well thought out (like the fight with two antlion guardians in the thumper-field. That was fantastic), and they go back to having time-outs every so often for the meta-plot to happen (I don't know about you, but my blood turned cold when the G-Man first turned up. Also, Eli's reaction to 'unforeseen consequences' is perfect).
I'm not saying that Ep:1 is a bad game, it's just a bit dull when compared to the ones either side of it.