While Rafa-Djokovic's 6 finals all had drama in them, I'd have to say Rogie vs. Djokovic USO SF was the best of 2011 and then in second place, Rogie vs. Djokovic in the 2011 FO SF.
I clearly remember sitting through the nearly 4 hour battle between Rogie and Djokovic and Rogie having the two match points, just like in the 2010 USO SF and blowing them both again and again losing 7-5 in the 5th set. Although Rogie lost both matches to Djokovic, they both proved how great the 29/30-year-old Rogie is, that one has to usually play the match of their life to beat him (i.e. 2008 W and 2009 AO Finals, Rafa over Rogie).
To sum it up, the 2011 USO SF was one the best battles I've seen since watching tennis for nearly a decade now (since early 2002 really). Rafa-Verdasco 2009 AO SF, Rogie-Rafa 2006 Rome F, they were classics too. Any match that lasts 4 hours or more is exciting, some great W matches too (2009 F Rogie over Roddick and 2011Isner over Mahut, 3 days and 11 hours, 5 minutes). Heck, even Rogie and Rafa's 2011 FO F was great, 4 sets, 3hours and 40 minutes and their best FO matchp ever. Seems like a lot of the great matches involve Rogie, Rafa, and/or Djokovic in some way, shape, or form.