For
Romania, it's quite hard to say who the greatest leaders were, because it was known by so many names and divided in so many states in the past 2500 years, and on top of that add that about 1900 years ago it's population and culture was totally revolutionized by the Roman Empire.
Going all the way back in time, to the time of the Kingdom of
Dacia, then
Burebista is by far the best Dacian king of all times. Though Decebalus is more known, he is more known because Trajan defeated him.
It's true, it was kinda impossible to defeat the Roman Empire at its maximum height under Emperor Trajanus, but don't forget even before the war begun, he still hadn't managed to put together a country even 2/3 of the one Burebista ruled.
During 106-271, Dacia was a Roman province, so the only leaders I could list here are Roman Emperors.
In
Wallachia (in Romanian called "Tara Romaneasca" so don't come telling me it's not Romania - the first one can be debated but this can't really be), the greatest kings IMHO were
Basarab I who founded it and defeated the Hungarians in the middle of their expansion in Europe, and
Mircea cel Batran (Mircea the Elder) who achieved some of the most important results of all of our rulers.
In
Moldova (another of the 3 Romanian principalities of the middle ages), there is no contest - Stefan cel Mare (Stephen the Great, aka Stephen the Great and Holy) is by far the most important king, having won 40 battles and losing 2! Including imposing huge defeats on the biggest empire of the time - the Ottoman Empire, again when it was in it's expansion.
In
Transylvania it's really hard to tell, since there were so many Romanians there that became kings somewhere else (like many Hungarian kings being of Romanian origin).
Mihai Viteazu, or "Michael the Brave" cannot be included in any of those though. He started as the ruler of Wallachia, but he managed to unify all the 3 Romanian principalities overpassing all the interests of the neighboring countries - which are not negligible (like Hungary, Poland and the Ottoman Empire, which were much bigger in that time than they are now). He conquered those and achieved the so-called "millenary dream" of uniting the Romanian principalities and most of the regions inhabited by Romanians. You could say he is the one who made Wallachia the dominant one in the 3 principalities, something which is still seen to this day (for example, the capital, Bucharest, is very off the center of the country, it's way south; why? Because it's in the middle of Wallachia, which is in the south).
Since Romania became a fully independent country with nobody having any right of messing with its business in any field at all, Romania had 4 kings (of which I met one, the only one alive today). I would not call the one who made the unification (that was not called a "king" - "rege", but a "leader" - "domnitor"), Alexandru Ioan Cuza, one of the best leaders of all times, but he still achieved very good things nonetheless. Of those 4 kings,
Carol I was the best, though
Ferdinand of Hohenzollern was not to be ignored either.
In the communist times, between 1945 and 1989, Romania had no good leaders. Period.
Since the revolution of 1989, the current president is by far the best of them all. Still, I would go back to the Monarchy any day. But that's another topic.
So if I were to post simply a list of names, in chronological order, beginning from 86 BC and ending with 2007 AD, I'd say the most important ones were:
Burebista, Basarab I, Mircea cel Batran, Stefan cel Mare, Mihai Viteazul, Carol I.
Note I did not mention one single king between 1600 and 1900, which can be partially due to my somewhat less knowledge of the period, but is probably due to the fact that there was not 1 single person to bring a great boost to the country, the good changes being brought rather a succession of good leaders than to 1 genius leader.