GURPS. But out of these choices, I suppose a system like SPECIAL would have to do. I would really prefer a system with no levels at all. Instead, completing quests earns you experience points that you can invest in skills, attributes and traits. And there has to enough meaningful choices of where to put experience points to make every character different - a pure mage must have different attributes, skills and traits then a fighter or a thief. This is one area where SPECIAL produced less then optimal results (it was way too easy to make a master of everything character), and where GURPS really shines.
Oh, and one other thing I would really like to see is something like GURPS disadvantages - negative traits that you can pick at character creation that limit you character in some ways, but provide extra points to spend on other things. This is another thing that allows different characters to be unique.