2009 by far.
AO - Rafa finally beating Federer in a hardcourt Slam; Verdasco breaking through and playing that awesome match against Rafa in the semis.
RG - Soderling stunning the world, taking out Rafa and making the finals; Federer finally completing the career Grand Slam.
W - Last stand of the New Balls generation; Roddick, Hewitt, Haas, Ferrero all in the last 8 (and Federer of course); Roddick's last big run and heartbreak in the final; Federer breaking the Slam record; Karlovic literally serving Tsonga and Verdasco off the court.
USO - Del Potro dethroning Federer under the lights in New York, after crushing Nadal in the semis; breakthrough run for Cilic.
WTF - Only final of a big event since 2005 AO with no member of the current top 3. Davydenko finally getting a big title; best WTF group ever with Federer, Del Potro, Murray and Verdasco - all matches went 3 sets, Murray out by one game.
Djokovic and Murray underperformed at Slams but still finished ahead of Del Potro in the rankings due to their fantastic Masters performances. They dominated the Masters circuit (apart from clay, where Nadal ruled).
Only top 10 member without a memorable GS/WTF run was Tsonga, who still had that amazing and memorable win over Federer in Montreal from 1-5 down in the 3rd.
Just an awesome year all around, my favorite year in tennis. Only thing that could have made it better was Roddick winning Wimbledon, also because we'd have had 4 different Slam winners.
This post says it all really and it's all true for me except the AO. I'll also add Madrid 2009 as a personal favourite of mine where the Fed drop shot made its debut and has been going (and getting stronger) ever since.