It may be debatable but let's look at some facts...Safin at his best barely beat Federer. Nadal at his very best made it hard for Roger to win GAMES.
Nadal and Djokovic are of course more consistent but you are selling them short to make it only about consistency yet to take once in a lifetime performances from the others (which usually didn't blow Roger out...certainly not like Nadal has and not even as Djokovic has beaten him)
That assumes that Federer was playing the same level, which is not the case.
Federer was imperial in AO 2005; it was surely one of his best ever Grand Slam tournaments in terms of level. He was just blowing everyone off easily, including Agassi who was dismissed in easy 3 sets in the quarters. He had something like 25-30 matches unbeaten. I don't remember the bookies odds, but only very optimistic Safin fans believed he could even trouble Roger in that semi.
FO 2008 on the other hand was one of the least convincing marches to the final from Federer, it was basically his aura that got him there despite not playing even near to his best.
It is not without reason that many regard Federer vs Safin of AO 05 as one of the best matches in tennis history, it was sublime quality from both. It was the real final of that tournament, no doubt Federer would have easily crushed Hewitt in the final had he made it.