Valve were back on form for Episode Two. Vastly different environments than we'd seen before, much more open-ended while still sticking to a tightly focused linear design, and it's honestly some of Valve's finest. Interesting new enemies and situations, great progression of the story (including an ending that made me cry) and much more creativity than Episode One. It's fortunate that this was so good, because it was unintentionally the last Episode, and possibly the last Half-Life game ever.
Looking back almost 10 years later, at least it went out with a bang, not a whimper. The gameplay, the world, the level design, is all Half-Life at its finest. It's just a shame there are so many intriguing questions raised during this episode, that we may never have answers to.