imo US Open 2008, not just because it's his 5th consecutive title, but as said on the other thread, because it happened that year, that he suffered some surprising losses during the season after his debut mononucleosis year, that he's mentally down the hole after RG's execution and the Wimbledon disaster.
His only titles then are .. Estoril on retired Davydenko and Halle vs Kohlschreiber ...
Well ... the only glimmer just before the US Open, is gold medal in doubles with Stan (with win vs Bryan bro in semi) .. but his start in NY is very laborious and not very convincing (Andreev ...).
This saves his season, because back problems will tarnish his end of the year despite the title in Basel, w / o Bercy vs Blake (the first of his career) and at the Masters, again the back, medical time out, physiotherapist massage vs Murray , with elimination in RR.
For me, US 2008 is really the 1st GS (excluding RG) where Federer plays his skin, because he is at 12 slam but the 14 of Sampras in these overall conditions seem very far away.
They were very close a year earlier after US 2007.
A year later, the deal is different, he plugged the hole, RG, took over Wimbledon, the No. 1, and the slam record, after Melbourne drama, more to come can only be a bonus.
He is almost too favorite in NY on heaven, his defeat then seems surprising vs. DP after goat mod vs Djoko, but relative, even if he will never regain his status in NY.