Is Bleach episode 161 filler?
Canon No, episode 161 of Bleach is canon.
#161
The Cruel Arrancar, Ulquiorra's Provocation
残酷な破面、ウルキオラの挑発
Aired 20 Feb 2008
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.