What is the proper title for someone who was a pope and is now canonized as a saint?
Sainthood can be liken to personal or professional achievement for ex. Peter Turner,
When he passed all the grueling test set by school and regulatory bodies, he will now be called Engineer, Doctor, Architect before his name acknowledging his earned reputation.
Engineer Peter Turner
Architect Peter Turner
Doctor Peter Turner
Now with regards to the Pope, same thing happen after they merited the crown of achievement that St. Paul described:
I have fought the good fight, I have finished the race, I have kept the faith. From now on the crown of righteousness is laid up for me, which the Lord, the righteous judge, will award to me on that day— and not only to me, but to all who crave His appearing. (2Timonthy 4:7-8)
Pope John Paul II after finishing the race was crowned in Heaven and was acknowledged by the Church here on earth to give honor to his faith witnessing. The crown of sainthood will be bestowed on his name, So the proper title will be the same as to those example above achieved in professional endeavors, this common similarities help us easily identifies their achievement. The title or crown will be write first. In the case of Pope John Paul II, he will now be properly called by his name adding the title or crown he won.
St. Pope John Paul II is the proper name as the Pope and his name are one by virtue of God's anointing. However some addressed or called him Pope St.John Paul II which also acceptable but this one is giving more weight on his position as the Vicar of Christ and the crown or title of sainthood appears to be only secondary. Why? All of us are called to become a Saint and all of us will be saint anyway when we enter the glory of heaven. But, the reality is, none of us can become a Pope unless God Wills it. So, the Pope position is higher calling than sainthood as it entails a greater responsibilities of shepherding billions of flock Jesus had entrusted to Peter.
So, by giving importance to the position held by the person as Papacy is a Divine Appointment not a worldly achievement or title we can all agree that the proper addressed would be Pope St. John Paul II.
But personally, I am more inclined to addressed him St. John Paul II the Great. We must not forget to attribute the additional title "The Great" as Pope St. John Paul II contribution to the Church is of great magnitude and all the world leaders visibly acknowledged his greatness surpassing all the Popes in Catholic Church 2000 years of history.
3 Reasons Why John Paul II is Called “the Great”
Immediately after St. John Paul II’s death, priests, bishops and even the next pope, started to give the deceased pontiff the title of “John Paul the Great.” While there are no particular criteria written down as to why a saint is labeled “the Great,” there are three virtues he possessed which gave him an extraordinary ability to imitate the example and teaching of Jesus Christ.
Cardinal Wojtyła’s “most distinctive characteristics [were] ‘his simplicity; his humility; absolutely his humility. It is something that draws you to him.’”
“He is first a holy man, a simple and humble man with a pleasant smile.”
So there you have it, humility, simplicity, holiness. Three virtues that make John Paul II worthy of the title “the Great,” because it is those attributes that most resemble the action of God in the world.
Three Reasons Why John Paul II is called Great
We can see that the Greatness of Pope St. John Paul II was acknowledge by the Universal Church and the World Leaders after his death.
But, we might be surprised that even satan and his demons acknowledge the greatness of Pope St. John Paul II The Great as he thwarted every plot of Satan to destroy the Church.
Satan arrogantly said in 1884 in Pope Leo XIII vision as he dared God that he can destroy the church in 75 to 100 years.
The vision of Pope Leo XIII
By 1959 the year Vatican II was announced and the 75th year of Pope Leo XIII vision, Satan smoke had entered the Church. So the year 1984 would be the end of 100th year if we immediately count the 1884 year as the starting point. But satan got desperate as he sees no hope in destroying the Church under Pope John Paul II as he was under the powerful protection of the Our Lady of Fatima "Totus Tuus". So, in 1981 satan had him assasinated but failed. So the 100th year had passed in year 1984 due to Pope John Paul II papacy, the arrogance of satan was thwarted by this Great Pope. This was confessed by the demons.
10 Ways John Paul II Defeated the Influence of Satan
Why Satan Is So Scared of St. John Paul II, According to Rome’s Chief Exorcist
In closing, the proper name in full to addressed Pope John Paul II by virtue of his acknowledged greatness by the Universal Church, the World Leaders and people in general plus most importantly the witnessing of the demons confessing his greatness to a famous Exorcist Fr. Gabrielle Amorth. Therefore all the faithfuls must properly and completely addressed him with the title:
"Pope St. John Paul II The Great". (Totus Tuus)