Constantine was the emperor of the Roman Empire who legalized Christianity and called the Counsel of Nicaea, among other thing. Which church traditions regard Constantine the Great as a Saint?
It appears that Emperor Constantine the Great is venerated as a saint in the Catholic Church, according to catholic.org:
Constantine was the most dominating figure of his lifetime, towering over his contemporaries, including Pope Sylvester I. He presided over the Council of Nicaea, gave extensive grants of land and property to the Church, founded the Christian city of Constantinople to serve as his new capital, and undertook a long-sighted program of Christianization for the whole of the Roman Empire. While he was baptized a Christian only on his deathbed, Constantine nevertheless was a genuinely important figure in Christian history and was revered as a saint, especially in the Eastern Church.
Catholic.org gives his feast day as May 21.
A page on the official website of The Antiochian Orthodox Church describes Constantine as a saint:
The Orthodox Church sees Constantine as the emperor who assisted the early Church in evangelizing the Roman Empire. For this reason it honors him as Saint Constantine Equal-to-the-Apostles [Constantinos Isapostolos].
Although I was unable to find an official statement by the Oriental Orthodox Church to the effect that they revere Constantine as a saint, the website of the Western Prelacy of the Armenian Apostolic Church of America, one of the churches in the Oriental Orthodox Communion, lists June 16 as the feast day of "Sts. King Constantine and his mother Helena". Thus, at least one of the Oriental Orthodox Churches considers him a saint.
The Wikipedia article on Constantine states that he is venerated as a saint by "Eastern Orthodox Christians, Byzantine Catholics, and Anglicans" (and in a footnote that he is recognized by the Oriental Orthodox Church), but I can find no specific backing for the "Anglican" claim from any reliable Anglican source. It also puts Constantine on a list of people celebrated in the Lutheran calendar, but notes that the Lutheran church "does not officially recognize saints".
My best supported answer, then, is that Constantine is recognized as a saint by the Eastern Orthodox Church, by the Catholic Church (particularly the Oriental sui iuris churches), by at least one of the (several) Oriental Orthodox churches, and perhaps by the Anglican Communion churches.
Constantine has never been added to the Roman Martyrology and thus has never been recognized officially as a saint. Nevertheless, Eastern Rite Catholics follow the Orthodox traditions and are permitted to do from Rome. His feast day is May 21. On a personal and private level Catholics are permitted to honor Constantine as a saint.