Celebrim
Legend
Based on what evidence?
Based on the fact that the author of the script doesn't seem to believe that Biggs is just talking Luke up, and more importantly on the internal evidence of the text that Luke shortly after this statement manages to show up and handedly defeat elite Imperial Pilots attached to Lord Vadar's personal squadron and Rebel Pilots who are "too welll equipped. They are more dangerous than you realize." Various other incidents in the text point to Biggs statement as being credible - for example the fact that Luke doesn't think that hitting a two meter target is particularly difficult even though it causes the other veteren pilots to groan or that prior to this he's been blowing up Tie fighters.
Biggs himself does better than most of the rebel pilots, managing to survive most of the battle only to be taken out by Vader himself. Vader is presumably a super-heroic character, so being defeated by Vader doesn't prove that you are incompotent. In fact, in narrative terms, if it takes Vader to defeat you then you are probably pretty compotent.
Even if Luke is the best, we know absolutely nothing about his competition. It's entirely possible that "the best... in the outer rim" is like being "the best football team in Scotland" - sure, if you're stuck there then it's better to be best than nothing, but as soon as you move to a larger stage, you're liable to be shown up very badly.
Considering that we are talking about thousands or perhaps millions of star systems, it's quite possible that Bigg's boast is a boast, but it isn't based on Luke's performance an uncreditable boast.
(Indeed, it's entirely likely that the only reason Red Leader doesn't mock Biggs' assessment is that he's desperate...
Perhaps. An even more creditable reason is that he already has seen Biggs in action, and therefore finds the assessment "He's better than I am." to be very convincing.
Alternately, immediately prior to this scene, Luke is easily ambushed by Sand People and beaten unconscious. Immediately after, he gets casually tossed aside by a thug who turns out to be nothing but a mook himself.
That sure sounds like 1st level to me.
It sounds to me like a level based system is a poor way to emulate the movies. If luke has 9D piloting (star fighters), it in no way effects his ability to avoid getting beaten unconscious by Sand People with 5D in Gaffi Sticks or taken down by a thug with 5D in brawling. Nonetheless, it's clear that whatever his melee combat ability is, Luke is far from a mook and quite capable of mowing down mooks by the handful virtually from the beginning of the movie.
Again, what do you base that assessment of Han on?
Han's backstory which, while perhaps exagerrated in non-canonical sources, is nonetheless creditable as a smuggler with a galaxy wide reputation based on hints from the movie that reinforce this assessment. Obi Wan seems to believe that Han is only boasting as his own account seems to incredible to believe, but Han backs up his claims about himself from that point on.