So, I have looked around a bit, but have no definitive answer, there are a few that could fit, but all of them also give me tiny doubts.
Albus Dumbledore, it would make lots of sense, seeing as he is one of the most powerful wizards ever, and the first person to acknowledge its existence, slipping details of it to Harry.
Rowena Ravenclaw: seems like something she would do, make a room that only the clever or the very lucky could discover, a room the only limits of which are the cleverness, creativity, and eloquence of the asker. Also, the diadem was hidden in there, giving her another connection to the room.
Godric Gryffindor: perhaps he got the same idea as Slytherin, to make a secret place where those of his choosing could gain an immense advantage over the rest. It also makes me think of the line made by famous Gryffindor Albus Dumbledore:
"Help shall always be offered at Hogwarts to those who ask for it"
Giving the room to those who are brave of heart and are of great need.
Go to this web address, and read this very interesting theory, that Lord Voldemort created the Room. While I think it is possible, I don't believe it for the reasons that this person gives, because their knowledge is very skewed, so read with a grain of salt.
As an afterthought, the idea of the three founders Gryffindor, Ravenclaw, and Hufflepuff being the co-creators is a very valid theory, giving evidence via symbolism, which is very important in the series, to be honest though, I would be very surprised if the answer to this question is never answered.