if I had to pick one, I would pick ronaldinho vs. milan in the san siro 2006 where he passed that magnificent milan team off the park almost by himself, barely ever giving the ball away and carving out a glorious assists for giuly's crucial winner. it wasn't just a great goal-getting/match-winning performance (ala rivaldo 2001) but it was a COMPLETE performance that lifted the entire team up and had us dominate. an absolutely magnificent performance vs. absolutely magnificent opposition (which is the key for it being top, milan were a great side).
the only one that comes close to it was xavi vs. germany in the 2010 world cup semi-final where he euthanised that outstanding german team with a glorious display of passing, and set-up the winner.
yes xavi vs. madrid in the bernabeu (in 2009, but also 2010) was more spectacular than either of the above, but IMO that madrid team was not as good as that milan team or that german team.
xaviesta in the 2009 CL final was a thing of beauty, as well.