The resurrection. Why? Because, (paraphrasing St. Paul) - without the resurrection, the crucifixion would have been meaningless! Seriously, the apostles pretty clearly made the resurrection the most important part of their message. The message of the resurrection means that Jesus must have died in the first place, because one cannot be raised from the dead if one has not died in the first place. And, it’s due to the resurrection that we even have a Christian faith! Had Jesus not risen from the dead in a glorified state, never to die again, His teachings would have died with Him. The apostles, who had given up everything to follow Him, could very well have given up all hope and returned home (Peter nearly did - recall the post-resurrection miraculous catch?). The resurrection is the cornerstone to the Gospel. No resurrection = no Gospel. The essence of the Gospel is, truly, Jesus took the sins of all on Himself at His crucifixion, then broke the shackles of death - reversing the curse laid on Adam for all who would believe in Him. As Jesus rose from the dead, so shall all who believe and follow Him also rise from the dead, and live eternally with Him, body and soul, in Heaven.