The worst for me is Berbatov to United followed by Shevchenko to Chelsea.
In mitigation Shevchenko went from playing in front of 3 creative midfielders to 0 creative midfielders - any striker would find this tough and he's proved that he plays well for footballing sides.
Drogba does well because he has the physical qualities to turn hopeful punts forward into assists but not many others would flourish there.
Shevchenko was also very committed (goal line clearances v Bolton and bleeding on the shirt in Sofia spring to mind) for all his shitness and this is where he really trumps Berbatov who just cannot be arsed.
The Berbatov signing was always going to be a bad one - yes they needed a 6ft plus forward but it needed to be one with pace and workrate.