Aside from the fact that hes dead, I'd say Haku had a mountain of potential.

He was brillient from an analytical viewpoint (strategy to counter Sharingan - note that his strategy for Zabuza was better than Gai's method of defeating the Sharingan - both strategies avoid the effects of being drawn into the power of the sharingan, however Haku's strategy also prevented the sharingan providing insight to Kakashi, allowing Zabuza to fight kakashi on 'equal footing' ie with the same advantages and disadvantages).

He created (as far as we know) 2 of his own jutsu of his advanced bloodline. He cannot have learnt them from someone else - there was noone to learn from. To this point, the only characters of around that age that have created their own jutsu is Neji (and likely Kabuto, but im going to ignore him for now). Point is, he's brillient with his jutsu as well as his analystical thinking.

Zabuza passed everythign he knew down to Haku, hence we can safely say that Haku is also very proficient at silent killing, in addition to what he learnt as an ANBU of hidden mist (we can conclude he was ANBU because that is how Kakashi explained he knew about acupuncture points). Knowing everything Zabuza knew at that age alone should be a big indication of his potential - if he can learn that much in the time it took for Naruto and co to become.... well... weak as Jiriaya puts it, surely he could develop far more in the future.

He instinctively knows the meaning of 'true strength' - one of the core themes when it comes to the character qualities that have been raised in Naruto - both the Hokage and Kakashi have mentioned it. Naruto barely grasps it, Sasuke is aware of it but apparently doesnt really embrase it, Neji is more concerned about fate and free will, Sakura is arguably clueless in this area, but Haku knows it from the depths of his heart.

On other characters:
Sasuke also has potential, but he needs to get over his blind furious hatred, or at least to turn it into a cold fury. Blind hatred results in rash descisions. Cold fury results in cold clinical calculations and strong determination...... but kinda turns the character into an evil kind of guy. However from a technical and efficent viewpoint, Cold fury may be the most effective state of mind.

Neji also has potential. Prior to the Chuunin exam, I'd say he was somewhere between Blind hatred and Cold fury towards the Head branch, but after it, he is presumeably over his anger towards the branch family (more or less). He is a genius (in the conventional sense) that surpasses sasuke, but if you match him up in terms of development to Haku, he loses out by a fair margin.

Edit: As an aside, I'd also like to point out that Haku is approximately the same age as the other Genin characters, and hence he had to have made it into the Hidden Mist ANBU some time before. This is approximately the at or even slightly before the same age that Itachi made it into Konoha's ANBU before making it to ANBU captain/commander (depending on translation). That should give you an idea of matching up his potential with someone else.

Additionally, if we are comparing characters at approximately the same age, it would probably be unfair to exclude Kabuto - I mean, in approximately that time (plus a year or 2 more) hes managed to become about as skilled as Kakashi (according to Orochimaru).

Final note: If Naruto magically acquired a set of working brains, then yes he would have potential. However as it is, the only reason he is going to get anywhere is because he is the main character, and hence he will eventually become rediculously powerful only by virtue of that fact (that he is the man character). Call me biased, but I believe the only way to know where youre going, is to have the brains to know how how you get there. If you dont, then you'll never get there. Naruto can say he's going to become the next Hokage all he wants, but unless his brains start working, hes not going anywhere except by virtue of his main character status.