Did Kirito lie about being the *best* beta tester? I watched SAO on Crunchyroll, where the subtitles for episode 2 have Kirito saying: 


  I made it higher than anyone else during the beta test.  The reason I knew the bosses skills is because I fought monsters with katana skills on floors far above us.


At the time, I just accepted this as "Kirito is just that good," which was born out by his skills and abilities later on in the series.  After all, someone had to be the best.

However, this question claims he lied during that speech, and I've since checked the dubbed version, where he says


  During the beta, I made it to floors that were higher than any of the other testers.  That's a fact.  I knew about the boss 'cause I fought tons of monsters with way more sword skills on higher floors.


The implication of word choice and tone in the dub does sound like the kind of exaggerated bragging that gamers do when they're claiming preposterous things, but the subs don't convey that to me at all.

Is there any information showing whether he was actually lying?  Was he just a good beta tester (possibly in the lead group), or was he really significantly above everyone else?
You can think that's a lie, but not totally.


  I made it higher than anyone else during the beta test. The reason I knew the bosses skills is because I fought monsters with katana skills on floors far above us.


There are two possible interpretations of his claim:


Kirito made it to higher floors than anyone else.

This is incorrect, because you cannot go to higher floor without defeating the boss first. And, after you defeat it, everyone else can go to the next floor through the teleport door.
Kirito have more experience than others.

This is probably correct, because from Light Novel Volume 1 - Chapter 2:


   The two months of beta testing were like a dream. At school I thought endlessly of my skill set, equipment, and items. I ran all the way home as soon as school ended and dived till dawn. The beta test ended in the blink of an eye. On the day my character was reset, I felt a sense of loss as if half of my actual self had been cut away.


Because of this, you can assume that Kirito had spent a lot of time on the game,  maybe more than others. Maybe he compiled more floor map information than other players... maybe, he knew more mobs, but he couldn't know mobs or bosses from upper floors. All information which Kirito knew was also available for other beta players if they tried to view it (maybe, but unique quests were unlikely to be made public).

There's no info about boss defeat in beta, but there probably wasn't any boss defeated by Kirito only (but that's my opinion).