I don't know. Spa 2010 was grim.... very
and that's not Alonso I'm talking about
Good point... Spa 2010 or Canada 2007... which one was worst?
Spa 2010 was one bad move from Vettel that ended his (and Button's) race, and gave him the hype of "can't overtake" (together with the not-so-gentle squeeze by Webber and his early chop in Turkey).
Canada 2007 was a sequence of mistakes, from lap 1 to the very end of the race...
China 2005 was a very painful show by Schumacher (crashing on his way to the grid...) but at least, the F2005 was a really bad car! (probably, the last truly bad car built by Maranello).
Hamilton has had quite a few poor showings, but I cannot recall a single one that stands out.
So anyway, I cannot decide which one was worst: the single, very costly move by Vettel at Spa'10, or the sequence of mistakes and erratic driving by Alonso at Canada'07. Probably still Alonso's. Unlike Seb, he was a 2xWDC by that time, and was a fixture of F1 by then. But it is a close call indeed.