The battle against Raging Raven was a little bit of a bitch, but more satisfying than Laughing Octopus because I always felt I was in danger instead of always feeling, "Where are you, bitch? Stop laughing."
The whole cutscene cluster

at the end of act 3 was actually pretty cool. Everyone from Liquid leaving Snake and Eva behind to the Holy Shit the System is His moment was really well done. Yay Kojima!
I pretty much wet myself when I got to replay the beginning of Shadow Moses Island MGS-style, complete with polygons and never-ending guards and security cams, oh my! And then going through the same area again MGS4-style was just as cool, hearing all of the voices/memories from MGS in the background.
I just blew up the GECKO using the MkIII to electrify his ass, btw. More progress today, hopefully.