It was almost certainly the other Ravenclaws.
First off, there's the obvious reason that in general, only Ravenclaws would have access to Luna's belongings. While it was indeed possible for other people to enter Ravenclaw Tower, as we see in Harry Potter and the Deathly Hallows, I find it highly unlikely that someone would risk getting caught in another House's area just to steal Luna's belongings.
At once the beak of the eagle opened, but instead of a bird’s call, a soft, musical voice said, "Which came first, the phoenix or the flame?"
"Hmm... what do you think, Harry?" said Luna, looking thoughtful.
"What? Isn’t there just a password?"
"Oh, no, you’ve got to answer a question," said Luna.
"What if you get it wrong?"
"Well, you have to wait for somebody who gets it right," said Luna. "That way you learn, you see?"
"Yeah... trouble is, we can’t really afford to wait for anyone else,
Luna."
"No, I see what you mean," said Luna seriously. "Well then, I think the answer is that a circle has no beginning."
"Well reasoned," said the voice, and the door swung open.
Harry Potter and the Deathly Hallows, chapter 29: "The Lost Diadem"
But in addition to that, it's worth noting that Ravenclaw House has a history of bullying and snobbyness.
In the old Slytherin acceptance letter to Hogwarts on Pottermore, there's this line:
For instance, we Slytherins look after our own – which is more than you can say for Ravenclaw. Apart from being the biggest bunch of swots you ever met, Ravenclaws are famous for clambering over each other to get good marks, whereas we Slytherins are brothers.
That shows already that Ravenclaws have a reputation for not really caring about each other. That's reinforced if you carefully read this part of the Ravenclaw letter:
Another cool thing about Ravenclaw is that our people are the most individual – some might even call them eccentrics. But geniuses are often out of step with ordinary folk...
So we can see from these that Ravenclaws tend to not really have the best intraHouse dynamics, reading between the lines.
And, of course, there's the case of Olive Hornby and Moaning Myrtle.
Both Myrtle and Olive were both Ravenclaws, and, as we know, did not get along well. Olive tormented Myrtle about her glasses - incidentally, paralleling everyone mocking Luna later on.
"Ooooh, it was dreadful," she said with relish. "It happened right in
here. I died in this very stall. I remember it so well. I'd hidden because Olive Hornby was teasing me about my glasses. The door was locked, and I was crying, and then I heard somebody come in. They said something funny. A different language, I think it must have been. Anyway, what really got me was that it was a boy speaking. So I unlocked the door, to tell him to go and use his own toilet, and then -" Myrtle swelled importantly, her face shining. "I died."
"How?" said Harry.
"No idea," said Myrtle in hushed tones. "I just remember seeing a pair of great, big, yellow eyes. My whole body sort of seized up, and then I was floating away . . . ." She looked dreamily at Harry. "And then I came back again. I was determined to haunt Olive Hornby, you see. Oh, she was sorry she'd ever laughed at my glasses."
Harry Potter and the Chamber of Secrets, chapter 16: "The Chamber of Secrets"
All in all, I think there's sufficient evidence that it was the other Ravenclaw students who were bullying Luna and stealing her belongings due to how odd she was. (For a related question on Ravenclaw and eccentric folks, see Ravenclaw House - is it really a good place for eccentric people?.)
