It should be pointed out that the Tetrarchy was shaky enough on its own. A case could be made that Constantius as Augustus had the authority to appoint Constantine his successor and the future emperor accepted a demotion to the lesser rank of Caesar. That same year Maxentius was hailed as emperor in Italy and defeated the eastern Augustus Severus. Galerius didn't help things by only promoting close friends - so instead of the Caesar Constantine stepping up to become the Augustus, he instead promoted another friend Licinius as Augustus - bypassing Constantine.

He did accomplish many things as emperor. One of which was simply reigning for 13yrs - itself is a massive accomplishment considering the constant civil wars that had been going on. Militarily he defeating the Gothic menace in 332 providing some (ultimately) temporary security on the Danubian border. He reformed the monetary system and brought some stability and halted the inflationary cycle by replacing the silver standard with a gold one (albeit abandoning the poor in an economic ghetto). He founded a new city of Constantinople (on top of the older Byzantium) and it's strategic and strong location would ensure the survival of the eastern empire into the 15th C. He also seems to have a very strong sense of justice and issued many rescripts addressing corruption in the empire - one of my favourites is from the Theodosian Code which preserves a law of Constantine where he states that if any of his officials has committed an injustice it should be brought directly to him and "I Myself will avenge Myself".

Was he OMG teh greatest emperor evar? Not in my opinion (and hopefully one of the flaired commentors will chip in with a more authoritative ranking) but he certainly was a very important one - he re-unified the empire, he was an absurdly successful general, his laws speak to his strong sense of justice and desire for proper and fair administration in the empire, and importantly he was a pragmatist. The latter is key considering the potential for another civil war if he had imposed his Christianity on the broader (majority pagan) empire. In his laws he states his distaste for pagan rituals yet he leaves it mostly intact (barring a few rituals).