30 Days of Heroes
I think a lot of people will tell you that this one is their favourite episode. I liked it for many reasons, those which include:
+ Nathan finally coming through for Peter
+ All the Heroes being in one place
+ Epic Sylar/Peter battle of Epicness
+ Niki finally getting the upper hand on Jessica
+ Micah and Molly being the cutest friendliest friends ever
+ Niki clobbering Sylar with a parking meter
+ The revelation that HRG's name was Noah
+ Epic blowy uppy brothers!
Other episodes that deserve mention in this catagory include Nothing To Hide (Cuz of Heidi, duh), Landslide, Fight or Flight, Building 26, Cold Snap and The Wall.