Aristotle,Metaphysics, 980a21, says that sight is above all others because we prize it for its own sake even when not immediately useful.
As we are animals, one would suspect that those most useful to finding food and avoiding danger would rank below sight in their order of usefulness.
Sight
Hearing
Touch-taste-smell seem equally important in this respect, so one would suppose ranking these would be a matter of individual preference or habit.
Thanks for the interesting question.

People can generally live pretty normal lives without the sense of taste and smell. Without sight or hearing, it's more difficult, but possible. Without a sense of touch, people would be pretty much unable to function safely at all. You would be unable to safely pick up objects because you wouldn't be able to tell if you were holding it tightly enough. You would be unable to detect whether the body was touching something that might cause injury (a hot surface for instance). Touch and pain are intertwined; you'd not know if you'd been injured. I suspect people with a total lack of touch sensation would have short lifespans for this reason.

I would rate them in order; Touch, Sight, Hearing, Taste, Smell (although the last two being mostly personal preference - I like food :-D)

We have many many more than 5. From a neurobiological and functional point of view, touch comprises several completely distinct senses, including vibratory sensation, light touch, and pain/temperature. Proprioception is a major sense -- it's the sense of where a part of your body is in space. It's how you know that your arm is above your head without looking at it. Balance is a complicated sense that detects whether your head is upright or not, what direction you're moving in, and if you are accelerating. There are many visceral sensations that you can be consciously aware of (visceral means specifically with reference to internal organs), like hunger, nausea, distension of the stomach or intestines, distension of the bladder, and irritative processes in the lungs or airways. All these things are neurologically different.

Finally, there are sensory processes that you're not aware of that are critical for your body to actually remain alive -- like sensation of oxygen tension, acid/base balance, blood pressure, and many others that allow the body to self-regulate.

Which is the most important? They're all important. You can make a case for all of them.

From my own ventures into Perception Phenomenology, I would give a fairly arbitrary list: Sight - Touch - Sound - Smell - Taste. We do seem to operate primarily by sight: when asked to describe something we often fall to categories of sight rather than the other four (although in specific contexts the others are used). When compensating for the loss of sight, touch seems to become the primary sense used (although I may be wrong, my experience with the blind is limited). From here sound seems like the next obvious step, as neither smell or taste appear eminently important. Between the final two, having no sense of smell (but having lost it) I can attest to the usefulness of smell over taste, as much of taste is smell but smell also serves other functions.

(From the perspective of a Chicken this all seems so clear, but how naive I am on matters of psychology...)

Lacking any of the major senses, you would be compromised in some critical way. Really, if you think about it, without modern society you would not have a crutch to lean upon. Modern society allows those lacking certain senses to experience life. Left to their own devices, sense deprived people would not survive long.

I agree with Aedes, there are far more than five senses, some of which we are not fully aware of their existence.

Given Theaetetus' agreeance with you, maybe there is more to your post then I was willing to give it.

For my (albeit limited) definition of senses, they have to be something we could be consciously aware of. Although we are not always reflective of what we see/feel/hear/smell/taste, when asked to comment on any one of those five we are able to elucidate what that sense relays (to varying degrees of success).

Your list raises two concerns for me:
1) How different are these sensations from the standard 5? To take an example, "Balance is a complicated sense that detects whether your head is upright or not, what direction you're moving in, and if you are accelerating." To what degree can we talk about balance without talking about the sensation of touch/feeling? I am not sure how balance would be possible without the sense of feeling: you 'feel' yourself moving towards one side or another, forwards or backwards, or accelerating in one direction or another. Just as an example, I cannot see how this would be seperate from how we feel within a space: acceleration is noticed because of the force we feel that holds us back, and possibly the air resistance we feel. Balance does not seem to be distinct from feeling, although a very strange use of it to be sure. This is something I see with many of the senses you offer, they all seem to be rooted in the other senses rather than in themselves as separate.
2) Is it possible to communicate these senses? This for me seems to be a large part of what we consider a sense. You mention several candidates for sensory processes which we are not aware, such as pH balance, which I would not rush to consider senses. If we are not aware of it, then how would we qualify it as a sense? I understand the body responds to it, accomidating for various homeostatic processes int he body without our consciousness of it (except in very strange cases, such as diabetics and insulin balance in the blood). However, I feel without consciousness of it, the ability to experience it as a perception, then it wouldn't qualify as a sense.

Perhaps we need a more rigerous definition of what we will call a 'sense'? My limited knowledge lends me to accept a more common-place definition, but this is apparently incapable of accepting alternatives. Aedes, have a better place to start for a definition?

(Which seems strange: all I sense is cold and a need for grain.)

Balance is an independent sense that does not rely on the other senses to talk about it. Spin around in circles for a while and then attempt to navigate. While it is not as obvious as the other five exteroception senses, when balance is defective an individual is greatly affected.

An individual also has interoception senses that deal with the internal body. Pain sensations are obviously not caused by senses that an individual normally would think about.