That's the first time I've seen someone question Fox's placing, and I'd be up for hearing a good argument as to why he shouldn't be first.
I think the reasons why he's so high up are fairly obvious. He has amazing speed, large profit from grabs, solid OoS/defensive options, a fairly decent recovery (especially when compared to his counterpart Falco), excellent gimping abilities (shine), amazing kill potential (usmash, uair, shine are the big ones), combos on every character in the top seven (I guess maybe not Jiggs, if uthrow > uair doesn't count). I think that Fox has the most potential out the cast to make the most out of any situation, due to (again) his great OoS options, and his excellent mobility. This lets him swiftly escape pressure, and handle being in a corner very well.
No one questions his versatility. I mean, we've seen over the span of the game that Fox has so much potential, so much variance with his moves. You'd be hard pressed to find a bad move among them (fair, I guess, would be the one). He can do a LOT off things out of his grab, the most notable being his uthrow > uair. With the strength of his moves complimented by his speed and rather good (albeit not controlling) projectile, he can switch between rush down and super camp really well, and really easily. There aren't many characters that can successfully change up their game this much.
His recovery. I've heard people say that it's bad, and it is in the sense that it has patterns that are relatively easy to figure out, but it still gives him a range of options for recovering. The length of his up B lets him ride walls much more effectively than Falco, and the length gives him more room overall to change up his angles and still be safe. It is not a one dimensional recovery like Sheik's or Marth's (or Falcon's). Illusion shortening is pretty good, too. How many characters can change the distance of their recovery within a few frames? Sure, Peach's and Jigglypuff's recoveries are excellent, but they don't have the advantage of recovering to the stage quickly, and their recoveries are pretty simple to figure out (Peach's more than Jigglypuff's, though). Fox can always keep his opponents guessing.
As far as combos and kill potential...shine. That's literally all I need to say. Shine spikes are a deadly threat for any character (except Jigglypuff, and to a lesser extent Peach), and leads into big moves on most of the top seven. He has other moves for combos and kills (utilt, usmash, uair, bair, nair, dair, etc.), but this is the most important.
That's how I see it, anyways. Sure, he gets comboed HARD, and edge guarded fairly well (that doesn't change the fact that his recovery is really versatile), but I don't think any character has the range of options Fox has. So...yeah, I guess it's theory, but so far, no other character has shown to be better.
Sorry, didn't mean for this to be so long.