"Give me your best piece of evidence for the resurrection", many atheists say. A similar question is "what would it take to convince you?" But the truth is, no one piece of evidence can convince most people of anything. The case for the resurrection requires many pieces of evidence, not just one.
In the header image, I illustrate this concept with the analogy of a jigsaw. In the first image, you see a complete jigsaw depicting Jesus walking out of His tomb. In the second image, you see just one piece of the jigsaw. The full image represents the full case. You can clearly see the picture the jigsaw shows. If we had a few missing pieces, but had the right pieces, we could still figure out the picture.
On the other hand, the single piece represents the "best" piece. Now, we could argue that this isn't even the best piece. Perhaps I could have chosen a part of Jesus' face. But then, Jesus is shown in many pictures, so maybe this isn't Jesus' resurrection, just like many atheists admit Jesus lived, but won't admit that He rose. But even if we say that I chose the best piece, a single piece is all it is. We can't reconstruct the picture on a single piece.
So we see that the evidence must be collected. But even when the case has been made, a mistake many atheists make is to focus on one single piece and say "that doesn't prove the resurrection". Take, for example, the empty tomb. Atheists can quite readily acknowledge that the tomb was empty, but they're actually right. Just because Jesus' tomb was empty does not mean Jesus got up and walked out of it. The empty tomb, taken alone, does not prove the resurrection.
However, most apologists will not rest their case on the empty tomb. Rather, the empty tomb is nestled in with a bunch of other evidence, often called some variation of the "minimum facts". That is, facts surrounding the Gospel that most historians, even anti-Christian historians, agree on. These facts, which can be individually waved off by a number of alternate explanations, tend to resist these alternatives when taken together collectively.
In my opinion, and it is an opinion in which I am not alone, the best evidence for the resurrection is the faith of the Apostles. Consider the words of Charles Olson: "I know the resurrection is a fact, and Watergate proved it to me. How? Because 12 men testified they had seen Jesus raised from the dead, then they proclaimed that truth for 40 years, never once denying it. Every one was beaten, tortured, stoned and put in prison. They would not have endured that if it weren't true. Watergate embroiled 12 of the most powerful men in the world-and they couldn't keep a lie for three weeks. You're telling me 12 apostles could keep a lie for 40 years? Absolutely impossible."
There are a number of ways in which atheists wave this off. Speaking specifically of Paul, for example, they say Paul was just feeling guilty because he killed so many Christians. First of all, this is a very weak, ad hoc excuse. Even if Paul did feel guilty for killing Christians, that doesn't explain why he would not only become one (he could have just started defending them), but actually suffer, and eventually die, for claiming that he, too, had seen Jesus alive. Neither would it explain his friends seeing the light Jesus caused when He appeared (which, if any of them denied, they left no record of). And it certainly doesn't explain why the other Apostles would just let him be one of them after everything he had done, rather than killing him in cold blood (especially since we know Peter was quite aggressive with his sword).
But aside from being a weak and ad hoc excuse, Paul is just one Apostle. The other 11 also held up the same testimony, most of them even dying for it. And so atheists have another cop out: Anyone can die for a lie. And this is true. I don't believe Islam is true just because so many Muslims are willing to die for it. But Muslim martyrs die/d for something they received. Not a single Muslim alive today saw Muhammad, much less the angel from which he supposedly received revelations. Even the Muslims who did see him reportedly challenged him a lot. His wife Aisha, for example, noted "I feel your lord hastens in fulfilling your desires" (Sahih al-Bukhari, Volume 6, Book 60, Number 311). So their willingness to die is based entirely on reception.
But the Apostles, and indeed a lot of the earliest Christians, weren't dying for something someone told them. They were dying for something they saw. You can't just wave that off as "anyone can die for a lie". They sincerely believed they saw the risen Lord. They gained nothing from spreading this message (unless they were telling the truth), and they lost everything (unless they were telling the truth), and so the evidence suggests they truly believed they had all independently verified that Christ is risen.
So atheists are forced to explain why 12+ people (there were other disciples, like Matthias and Barnabas), including a former persecutor of the Church, were all willing to die for the belief that they had seen the resurrected Lord. This is, in my opinion, the best evidence for the resurrection. But it is the best piece of evidence for the resurrection. On its own, it doesn't prove the resurrection. It's nigh impossible to explain, but if, say, the Pharisees discovered Jesus' body and paraded it around Jerusalem, the testimony would have been invalidated, and the question of why these men were willing to die for it would be completely irrelevant.
But of course, that didn't happen. There are plenty of other facts surrounding the resurrection, none of which are enough to single handedly convince someone to change their worldview, but all of which, when combined, very strongly indicate that the resurrection happened. And that's why atheists, or indeed other unbelievers, typically don't like to take them all together. It's also why, whether they are taken separately or as a whole, they never commit to alternative explanations. It's always "well maybe..." to explain one piece of evidence, it's never "actually, this explanation fits all the facts better". The truth is, when all of the facts are taken together, the only explanation that fits them without significant flaw is that Christ is risen.
And that's good news. If Christ has not risen, the Christian faith is vain, but if Christ has risen, as the evidence so strongly suggests, then the sin debt of the entire world has been paid. Because of Jesus, every single member of the human race can approach Him in faith and ask for forgiveness, and receive it. Because Christ is risen, the Church is forgiven. Believe, repent and receive.