In what way is the Resurrection the best explanation?

[Vicki Q]

They were able to find select passages like Isaiah 53 that could be reinterpreted in that light, but the fact that there are no passages in the Old Testament clearly prophesying that the Messiah would become the ultimate lamb to be sacrificed for all our sins is telling. There's not even a passage in the OT that specifically says the Messiah would die and then be resurrected.

That's Jewish apocalyptic prophecy for you. It's a jigsaw puzzle you do after the events have given you the picture to work towards.

Again, the non-Christian Jews (from what I've read) don't accept that Isaiah 53 is about a person, let alone a Messiah.

Check out Wikipedia for counterexamples, but I must stress Isaiah 53 applies to both a nation and a person acting on behalf of the nation.

Incredibly biased sources, and scant/undocumented relevant details in contemporary "neutral" records, means we can't really convincingly speculate what exactly happened.

Whether the sources are evangelistically biased or not, they do tell us very reliably what the Early Church believed.

Vicki, come on, you're intelligent enough to know what's wrong with this statement.

Yes, nothing is wrong with the statement.
If you want to know what the Liberal Democrats believe, you read their manifesto. Sure it's very biased towards the LDs, but it tells you very accurately what they believe.
If you want to know what C1 Jews believed, read Josephus. Sure it's very biased towards C1 Jews, but it tells you very accurately what they believed.
If you want to know what the Early Xians believed, read the NT. Sure it's very biased towards Early Xianity, but it tells you very accurately what they believed.

Bart...But if you want something more concrete, here's Tim O'Neil addressing the Resurrection specifically:

Could you unpack the Bart Ehrman explanation a little, please?

As for T.O'N piece, (BTW a lot of what follows also applies to your earlier construction and I think BE as well):

There are a significant number of  errors in this and it would take too long to deal with them all. From Mark “generally dated to after 70 AD” (e.g. Wikipedia, Britannica and the actual consensus have <70); through his inexplicable misunderstanding of Tom Wright on the 'Resurrection as inauguration of the KoG' vs 'purely return from the dead miracles' (hopefully careful readers of this thread will know the very clear difference!); to his use of the 'spiritual bodies' argument (he's read Tom Wright on what the words mean, why is this still happening?).

More importantly he's missing what Paul is saying. In 1 Cor 15 Paul uses the non-physical word for appearances because that's exactly what Damascus Road was. Paul is writing 1 Cor 15 to establish his status as a Resurrection witness, which gave him special status within the Early Church. As such, he has carefully chosen a word that covers all appearances in the list he wants to be on.

All sense of ambiguity vanishes in his use of anastasis and egeiro which are only used of non-bodily resurrection. Furthermore, the C1 Jewish conception of Resurrection that Paul talks about at length involves physical bodies. Therefore the 'evolution' to a full blown uncontroversial and physical Resurrection must have taken place long before Paul wrote at around...the time of the Resurrection. So no evolution. Just an event.

However the much repeated problem still hasn't been resolved. 
He doesn't explain why the disciples kept the whole thing going, let alone proclaimed that Everything Has Changed.
He doesn't get us from the dead wannabe Messiah waste man to the Pauline embodiment of God who has inaugurated the KoG.
He doesn't explain why the Earliest Church concluded that death had been defeated. Seeing a vision of someone who's died is a pretty conclusive indication that they're still very dead. But the disciples were clear on this- Jesus was alive!

So given the belief, what caused it?

We don't know. That's the point I'm trying to get people to understand.
At the same time, we can still offer potential accounts of what might have happened, in spite of the lack of evidence and relevant details,

But none of the accounts put forward are plausible. Given centuries of high powered scholarship has failed to come up with a sensible alternative, perhaps the time has come to say the disciples actually knew what they were talking about?