Prompted by some writing by Stewart Mandel below, I looked a little further into this. In the top 25 (the AP, Coaches and Harris versions), there is a combined total of six losses. Yes, six. That's six different teams in the top 25 with a single loss. LSU has inflicted two of those, 33 percent. If you ranked the teams with losses (based on the poll results), the Tigers have beaten two of those top four. One of the other six losses came to a team LSU beat. I know it's a Six Degrees of Kevin Bacon, but that's half of the top 25's losses.
As for the best conference in the country, the SEC has four of the six wins over top 25 teams this year. LSU has two while Georgia and South Carolina each have one.