This is obviously a highly subjective topic that could have several possible answers. However, in my view, there are really only three people who could possibly be considered the greatest Christian ever, and those three people are Peter, Paul, and John. When you look at their accomplishments and the things that are written about them they stand out far above the rest.
You could make a very strong case that either one of them was the greatest Christian ever. Of course, it depends on how you define great (as in more powerful, leadership ability, evangelism, theological ability, etc.) but, in my own opinion, if we add up everything in Scripture Paul will come out slightly ahead of Peter and John. I will tell you the reasons why I think Paul was the greatest Christian ever, but first let's take a look at their respective resumes:
Apostle Peter
- leader of the original 12 apostles
- name is a foundation of the New Jerusalem (Rev. 21:14)
- probably dictated the gospel of Mark
- wrote 1 + 2 Peter
- performed miracles/signs/wonders (Ac. 2:43, etc.)
- healed the lame beggar (Ac. 3)
- Ananias/Sapphira fell down dead at his feet (Ac. 5)
- his shadow could heal the sick (Ac. 5:15)
- healed the paralytic (Ac. 9:33-34)
- saw angels + they rescued him from prison (Ac. 12:7-11, Ac. 5:18-20)
- saw visions of God (Ac. 11)
- resurrected the dead (Ac. 10:37-41, Mt. 10:8)
When we look at Peter's resume we can see why some may consider him to be the top Christian of all time. But let's look a little further:
Apostle John
- one of the original 12 apostles
- highly favored by Christ + was "the disciple whom Jesus loved"
- wrote 1+2+3 John, the gospel of John, and Revelation
- name is a foundation of the New Jerusalem (Rev. 21:4)
- performed miracles/signs/wonders (Ac. 2:43, etc.)
- healed the lame beggar (Ac. 3)
- rescued from prison by an angel (Ac. 5:18-20)
- caught up into heaven to see visions (Rev. 4:1-2)
- resurrected the dead (Mt. 10:8)
Clearly John has an incredible resume. One of the major things that separates John from Peter is the fact that he was caught up into the heavens (quite an impressive achievement) while Peter was not. And finally:
Apostle Paul
- Christ appeared to him in a vision on the road to Damascus (Ac. 9)
- did many signs/wonders/miracles (Rom. 15:19)
- struck a false prophet with blindness (Ac. 13:8-11)
- healed a lame man (Ac. 14:8-10)
- could heal diseases (Ac. 19:11-12)
- resurrected a dead person to life (Ac. 20:9-10)
- saw a vision of an angel (Ac. 27:23)
- unaffected by a poisonous snakebite (Ac. 28:3-6)
- was caught up into the 3rd heaven (2 Cor. 12)
- wrote more books of the NT (and of the Bible) than anyone else (13 in all)
- is the focus of Acts 13-28 (the majority of the book of Acts)
- may have been responsible for much of the gospel of Luke
So here we have Paul who, like John, was caught up into the 3rd heaven. This is obviously something that both of them have that Peter does not. However, one of the serious drawbacks with considering Paul the greatest Christian ever is the fact that he never physically followed Christ while he was on earth. That is something that both Peter and John have over Paul.
Anyways, you could make a very strong case for either of these three being the top Christian ever, but I chose the Apostle Paul because I believe that he was the strongest overall. He was very good theologically and probably actually the best ever when it came to theology. He was just as powerful as either Peter or John as far as signs/wonders/miracles were concerned. He was probably the greatest evangelist ever and also one of the top leaders in the early church (if not the top leader). Peter was never caught up into the 3rd heaven, as both Paul and John were, which is a serious drawback. John was not quite the leader or evangelist that Paul was. So overall, my list would go as follows:
1. Paul
2. John
3. Peter
I would place John ahead of Peter since John was caught up into heaven and Peter was not. John also wrote more of the NT than Peter did. Anyways, that is my list. Feel free to rearrange it
EDITED TO ADD: First, Christ is not on the list since we are only considering candidates who are fully human and were born of both a father and mother. Second, please do not be hyperliteral with the word "greatest" and turn this into a discussion on who was the most humble of all. By "greatest" I mean the most powerful theologically, the best evangelist, the most powerful miracle-wise in the Holy Spirit, the greatest leader, etc. You could also consider it a requirement to physically follow Christ if you wished. Thanks