This season has been rather underwhelming. It's had its moments, but no standout episode yet.
The UN one would be season 3. If you really want to get into Community, try these episodes:
"Contemporary American Poultry"
"Advanced Dungeons & Dragons"
"Conspiracy Theories and Interior Design"
"Remedial Chaos Theory"
"Pillows and Blankets"
"Digital Estate Planning"
My favourite's gotta be Remedial Chaos Theory. Alternate timelines, chaos, character studies and other fun stuff.