Okay, here's six from each season. I tried to get ones that show off different cast members as well as being funny.

Season 2
2x04 Practice Date - Really good Leslie and Ann episode. They go on a fake date!
2x08 Ron and Tammy - Guest starring Megan Mullally! Also a partial explaination for why everyone hates the library.
2x09 The Camel - This one isn't the best ever but it has one of my favorite Tom moments.
2x10 Hunting Trip - First episode I ever saw. It's pretty great as well.
2x15 Sweetums - Has probably my favorite Town Hall sequnce of the series.
2x17 Woman of the Year - Great Leslie and Ron moments, also the first appearance of Jean-Ralphio.

Season 3
3x04 Ron and Tammy: Part Two - More of Tammy, the librarian from hell.
3x05 Media Blitz - The source of "Ben Wyatt: Human Disaster"
3x07 Harvest Festival - The first appearance of Li'l Sebastian.
3x09 Andy and April's Fancy Party - Super sweet and has great Donna and Ann bonding moments.
3x13 The Fight - Best drunk acting of the whole series.
3x16 - Li'l Sebastian - The trimphant return of everyone's favorite minihorse.