1. Act 3
2. Act 4
3. Act 2
4. Act 5
5. Act 1

I loved the atmopshere of Act 3, it really made it enjoyable for me. Act 1 was pretty slow, not much really went on.

***FILLED WITH SPOILERS---READ AT YOUR PERIL****


Wow, that's a tough one! It's changing on my second playthrough gameplay-wise because I'm attempting no-kill-all-stealth this time. But based on my first playthrough:
1. Act 5- The final boss fight was one for the ages...Psycho Mantis is defeated without a shot being fired--by the PS3...Crawling on the floor thru the microwaves, smashing that triangle button was probably the closest I came to feeling Snake's exhaustion--my arm was about to come off...The last B&B battle was genius...all of the cutscenes...the ending was perfect
2. Act 4- Sniper Wolf hearts Snake...All of the ghosts of MGS past...Rex fight...'Switch to disc 2'...Intro to act 4 via MGS1...Naomi? Naomi?!? NAOMI!!!!!...Vamp vs Raiden in splitscreen
3. Act 3- Otacon gets some...Big Mama's big reveal...The Triumph's triumphant return...best chase scene I ever played...awesome boss fight... Liquid's Act 3 outtro was pure badassery
4. Act 2- Another great chase scene...Monkeys smoking...Snake's a WMD?...Fantastic environment...the Colonel likes French rations...Vamp's back...Snake drops his cig and sneaks a peek(can't blame him)...Jungle tracking...Raiden ressurected as a BAMF
5. Act 1- Hello MK II!...PMC's must die...Drebin says hi and*burp*...FROG fight!!...'I can't believe I'm playing MGS4!!'...Next gen has been good to Meryl...Surprise guest from MGS with IBS...Liquid's intro

Yeah, I guess it got better for me as it progressed. I really can't say enough good things about this game. I laughed, I got all misty eyed, got nostalgic...but mostly, I was utterly enthralled by just about every minute. Kojima even got me to feel sorry for Vamp at one point. Vamp! All of the Acts are genius in their own way. Really, they're all about equal to me. Kojima has just sold at least one copy of every game he ever makes from now on.