Mostly Dean, some Sammy and a little John for good measure.
Dean as Michael's vessel
First off, how much do I love the fact Dean is Michael's vessel? A whole damn lot. I think we pretty much always knew Dean had something to do with Michael the Archangel (since S2's Houses of the Holy) and there's been a bunch of speculation regarding it (Anna-type deal with Michael being reborn as Dean etc) so thank the Kripke for finally saying what's what.
And also, bless Dean for being all 'eat me' about it. Nobody's riding in that meatsuit except him.
We've seen as far as vessels go, there are pretty specific rules in Supernatural. Angels can only possess certain people and only certain people can also hear angels (Anna, being a fallen angel herself or people like Jimmy/Jimmy's daughter/Nick). As we found out in Lazarus Rising, Dean's not one of them ("Certain people, special people can perceive my true visage. I thought you were one of them. I was wrong." - Castiel).
But he is special. Him breaking in hell was the first seal and as soon as the angels found out, they 'laid siege to hell' to get him out. But I wanna talk about this later, Michael's first.
In Sympathy, something pulls Sam and Dean out of Lucifer's way and brings Cas back from the dead. Now, I don't think that was God. I think Zach was telling the truth when he said God has left the building. And also, I don't think it would make any sense (but I could be wrong) because if God is back, he could pretty much clear up the entire mess right now, being well, God. I think it was Michael.
Anna told us only four angels have seen the face of God. In lore, those are the four archangels (Michael, Gabriel, Raphael and Uriel, I think. However, Kripke's Uriel isn't an archangel, but that's Kripke for ya.). I don't know how many archangels there are in Supernaturalverse, but Michael is definitely one of them. And there's the one watching over Chuck (Raphael from 5.03 sides, I guess). That makes two. We know Chuck's angel is doing his job, maybe Michael did his? He can't come down on Earth without a vessel, but he can make damn sure the vessel stays intact.
I think (and this is purely my speculation/wishful thinking, partly because I'm iffy about God) when the angels realized the first seal was broken, Zach decided to roll with it and may have done something to the archangels (or, at least Michael)so they wouldn't stop Lucifer from rising. Maybe Michael has gone along with God on the vacation, but came back early to pull Sam and Dean out of harm's way (and also bring Cas from the dead).
Michael isn't down with the apocalypse/came back too soon/found out about Zach's enema (cough) and now Zach is freaking out because he's in trouble. Otherwise, if Michael had been here all along, I don't see a reason why he couldn't do what Castiel and Lucifer both did to their vessels (talk them into it). Now that Lucifer is out, Michael *has* to do his job (stop him) and he needs Dean alive and breathing (seeing how Dean nearly got killed a lot through season four and Michael did nothing tells me Michael wasn't around until recently). There's also the question *why* Zach and his posse did nothing to get Sam and Dean out of that church.
The Prophecy
And it is written that the first seal shall be broken when a righteous man sheds blood in hell...
The man who broke in hell is Michael's weapon to defeat Lucifer. But the thing about prophecies is, there are many different ways to interpret them. The prophecy (the parts we've heard so far) says, whoever breaks the first seal is the only one who can stop it. The apocalypse. So when Dean breaks, the angels better get him out fast (before he turns into a demon) or they're pretty much screwed.
I wonder, how much did the demons know? Alistair said they thought it was John. I think, initally, they did for whatever reason (it could be the whole 1st seal/vessel thing is blood-related, just like regular angel vessels), which is why Azazel made the deal with John (John offered the Colt, Azazel wanted John's soul as well) and Dean was 'allowed' to live because, as far as Azazel was concerned, he was not important in the big picture (for that matter, I don't think John's deal went through Lilith - Azazel simply made the reaper do it, even though she didn't want to). So, John dies, goes in hell and whoops, he's not the guy.
I think as soon as John's soul got to hell, the demons realized it wasn't him. Which is also why he was able to get out when Jake opened the gate. You'd think if he really was that important, they wouldn't just let him hit the road as soon as he could. Also, I don't care how awesome John was (serious lack of parenting skills, but the guy was still pretty awesome), there is no way anyone can spend a year in hell (which would be what, over a century in hell-time?) and not break, especially if the demons' entire plan depends on it. I think what Alistair did to Dean was very much special treatment to make sure he broke and broke fast.
Which also make sense in AHBL, when Dean makes his deal. As far as demons were concerned, they got what they wanted. Azazel got his winner (Jake) and the gate was about to bust open (and Lilith could walk out). So they didn't need Sam. But they needed Dean. And he walked right into it. He offered his soul, the CRD gave him one year* and made sure he wouldn't try to get out.
A year later Dean dies, Alistair throws him on the rack and does his thing, the angels finally get a clue but are too late (or rather, someone *cough*Zach*cough* didn't deliver the news on time) and the first seal breaks.
*I think this is part of the red tape. Just like Azazel couldn't get to Sam and the other kids without Mary saying yes even when she didn't know what he wanted (just like Nick doesn't know Lucifer wants to end the world. Manipulation is the key.), if Dean hadn't offered his soul, they would have nothing. If Dead died without selling his soul, he'd be just another poor bastard in hell.
But as I said earlier, there are different ways to interpret the prophecy. Ruby said to Sam it always had to be him (to kill Lilith and break the last seal). Lucifer told Azazel in 4.22 to find him a child. A special child. One. Azazel was searching for that one child. I don't think he made the deals randomly. There was probably something all the Psykids had, something that I think is similar to angelic hosts. Like we saw in The Rapture, it's a family thing. Jimmy's daughter was able to be Castiel's host. Maybe it's sort of the same thing with the psychic children. Azazel didn't give them powers by feeding them his blood, he just made whatever it is wake up. Like Ruby said, you don't need the feather to fly. The potential has always been there, they just needed to be shown the right (or wrong) way and take it.
Brothers
The family theme is by far my favorite thing in the entire show. Forget the urban legends and apocalypse (as much as I like both), this is a story about brothers.
And the family plays a really crucial part in everything. Sam and Dean come from a hunting family. If John hadn't become a hunter after Mary's death, they'd have normal life. Even with Azazel's plan. Sam would simply disappear one night and no one would know what happened. And Sam would either win the Psykids showdown or be killed by Ava/Jake/someone else. And that part of Azazel's plan would still happen. Only, if Sam had died, Dean wouldn't know anything about it or crossroads demons and wouldn't sell his soul to bring him back.
Azazel (or whoever) would need to find the 'righteous man'. And I think, just like Jake or any other Psykid could take Sam's place, someone else could take Dean's. Again, the potential - any of those children could potentially be 'Sam' and the same goes for Dean. Other people could potentially be the First Seal/Michael's Sword, but it turned out to be Sam and Dean (and in a way, John) who made the right and wrong choices to be where they are now.
John made the choice to die instead of Dean. Dean made the same choice regarding Sam and when he was in hell, he made the choice to get off the rack (and who the hell wouldn't with Alistair cutting them up 24/7 for thirty freaking years). Sam made the choice to trust Ruby over his brother (as much as it breaks my heart, I don't blame him. He was desperate and stubborn and so much Sammy it hurts) and learn to use his powers and kill Lilith.
They were both manipulated by heaven and hell, but they did make those choices. Free will is a bitch. And it's not their fault the apocalypse happened and Lucifer is out. Whatever they did or didn't to, they had no idea of the consequences. Just like Mary had no way of knowing what bringing John back from the dead would mean in the great picture.
Dean didn't know what hell would do him or what he would to in hell and until the last moment, everyone believed killing Lilith would stop the apocalypse. Yeah, Sam was stubborn and he did trust a demon over his own brother and that is his fault. But he, like John, was driven by revenge. Lilith killed his brother and she had to die.
This came out way longer than I intended, but here it is. Just so I can be Kripke'd :)