I spent so long trying to edit my post that i couldn't anymore, so...: Here's about half of what i meant to post. (put this between the two paragraphs and you get what i meant to say...)
My final assessment of him would be: Nearly limitless potential, and fighting ability when unleashed, seems to hold himself in check much of the time, so that ability does not come through. He came into that ability between the JA series, (being stalemated by desann) and the beginning of NJO, continued to progress up until his KNOWN "coming out" in TUF, and has increased in ability almost exponentially throughout Dark nest, and LOTF. He is now to the point of peerlessness in the CURRENT star wars universe that authors decline to let him clean up the messes anymore, starting with TUF, and continuing through LOTF. (Though he was the combatant who defeated Raynar Thul (without showing too much effort, if i may remind everyone)
We obviously DON'T know the extent of his force knowledge, though it is doubtful he knows as much as the masters of old, who were actually taught it, it is possible he knows much that they never did. Remember, they thought of the shatterpoint ability as a "Gift" (in Shatterpoint) while Luke has learned it is a taught skill that he taught to Jaina in an afternoon. The author's also hint there is much we don't know about his knowledge when they show Jaina's surprise at what he can do. They never explain exactly HOW he disappeared from the force, convincing everyone he was dead, remember, the Jedi knew when someone had just left the force,(Ben, Mara, Jacen, etc) what Luke did was different (Even Jacen thought he was dead, even Leia, his twin)
They never explain his force illusions that so badly misled Jacen( NEVER seen before, by ANYONE in the star wars universe) (i'm talking about his rewriting Jacen's visions to be whatever luke wanted him to see) So luke's force Knowledge is at least ON-PAR with ANY KNOWN knowledge, though it may not be the same knowledge possessed by the ancient sith and jedi. Different is not necessarily less.
I'll point out again that luke is the only top tier force user we still have who is currently undefeated. Every one of the ancient sith, and jedi were ultimately killed by SOMEBODY. (or yoda, defeated, forced to retreat) So the force is obviously with Skywalker, who plants himself in the middle of Galatic Conflict after Galactic conflict, probably more such harm way than any known force user in the entire history of the galaxy.
(the only way you'll change my mind on that is if you can count up more conflicts for some other character than there have been for luke, and you won't. Now you might say its because he's the main character, so the author's are biased. That won't matter, because what the author's say is canon, whether it is fair or not.)
So in conclusion: I would give Luke the top rank in the star wars universe, at the end of LOTF by a decent margin. That margin can be discussed fairly only after discussing other characters however.