I always loved the one where Ross and Rachel went on their date in the planetarium... "oh honey, that's ok!" I laughed so hard!
The one where Chandler's in a box is a classic, and speaks to the friendship between Chandler and Ross:
But I don't think it's possible to top the one with all the Thanksgivings! I love pretty much any episode with a Monica flashback...
What are your favorite Friends episodes?