As for Aragorn : we don't know much about his reign. His reign must've been quite peaceful though, in comparison with previous ages, as Sauron and his minions were obliterated. There were only some battles with Easterlings and perhaps Haradrim, but compared with Sauron this was rather a piece of cake. King Elessar didn't have to deal with any major problems. King Elessar is just as good a King as the good Kings of Númenor and Gondor.
As for Théoden : during most of his reign he was deceived by Saruman, and during the events of the WOTR completely controlled by Gríma. Yet, he was given a chance to redeem himself, which he gloriously did (although one can argue that he was just overcompensating for the fact that he had acted as a slobbering, lethargic Methusaleh for most of his reign). For me, there were Kings of Rohan that were greater than him : Éorl and Helm Hammerhand, for instance.
As for Elven kings : hard to say. Finwë didn't do anything special, safe remarrying and taking side for his firstborn, whilst his sons were all great in their specific talents, but seemed to lack in others.
As for Fingolfin's sons : Fingon was similar to his father, whilst Turgon was wiser than his father (although not wise enough to leave Gondolin).
All in all, it's a difficult question to answer. If Finrod had been a King (and with this i mean High King, not just Lord of Nargothrond), i'd pick him. But as he wasn't, i'll take the safe road and pick Manwë, King of Arda, as the wisest, noblest and mightiest King...
My 2nd greatest King is Ar-Phârazon.
Why? Because in his own way, Ar-Phârazon did some great deeds. Of course, he was deceived by Sauron, but he besieged the Valar which is still a very courageous act, even though it's utterly foolish - as foolish and as courageous, as Fingolfin challenging Morgoth.
Also, the fact that Ar-Phârazon was deceived/controlled by Sauron, can be compared with Théoden being controlled by Wormtongue - the difference between the two is just that Ar-Phârazon didn't have a White Wizard to help him. No one could help him, unless the Valar would send help.
He's not a villain, but only one of the biggest anti-heroes of Tolkien's works, after Gollum, Túrin and to a lesser extent, Boromir.