1x07 The One with the Blackout [goodness-knows-what-th watch]
Probably the most easily-recalled episode since the pilot, but personally I don't think it's the funniest episode thus far (despite it's rating compared to the others on TV.com, linked above), more a memorable setup. Still, at this point in its run, "lesser Friends" is a purely relative term.
3x01 Castles in the Air
[Watch it (again) on ITV Player.]