No they aren't the best team ever. But they are the best team this year.
Let's look at their games. Miami they gave up 10 points, Tennesee they gave up 6 points, LSU they gave up 21, Kentucky they gave up 5 points, Georgia they gave up 10 points, Vanderbilt they gave up 14, FSU they gave up 15
Now lets see how many points they scored in those games. Miami they scored 26 points, Tennessee they scored 30 points, LSU they scored 51 points, Kenucky they scored 63, Georgia they scored 49, Vanderbilt they scored 42, FSU they scored 45.
Good defense beats good offense.