Always had an interest in the Romans but particularly Constantine as I am from York, UK and he has a big connection to our town! I would love to know what make the 5 so great and what particularly sets Constantine apart. From my (light) research he doesn't seem particularly different.
EDIT: I should add that this explains the 5 Good Emperors in the official sense as a dynasty. But most writers seem to take these as the best, why not Constantine? I know he is later.
Asides from not falling into the dynasty of the five good emperors, Constantine left a complete mess in terms of his succession, and divided the Empire once again amongst his sons immediately after uniting it. Constantine's succession issues are likely the biggest thing that hold him back from being one of the good Emperors I think when listing top emperors of Rome, there are numerous better choices than Constantine, and thats probably why he doesn't make the list, he wasn't one of the worst, but not one of the best either.
Edit: Id also like to add that Constantines place chronologically likely doesn't help his reputation very much either, he came immediately after Diocletian, one of the the strongest emperors, and contributed to the downfall of the Tetrarchy, which had brought stability and ended the crisis of the 3rd century.