Chiikawa Pyjama Parties Arc is the GOAT
The most important aspects of Chiikawa to me are the characters Usagi and Ponchette, I love to see Usagi be tuff and witty, and seeing Ponchette interact with the Chiikawas.
My main reasoning for pyjama parties being the best arc is that Usagi had so many tuff and hype aura moments, from the start when he jumps from a tree and so confidently sings his Chiikawa Parties no Uta rendition, when he burst onto the stage whilst chiikawa was so worried, and when he defeated the giant call duck and saved the chiikawa parties group with his nonchalantness, bravery and wit.
Another great part is the focus on Ponchette no Yoroi-san, who is my second favorite character, he was the creator of the pyjamas and it is always nice to see how happy he is when the creatures use his crafts. The pyjamas are also a very cute outfit and the song is good, which increases the aura further.
The pyjama parties arc takes place between episodes 144 - 155, but the pyjama outfits debuted in episode 46 when they were gifted to the main trio by Ponchette no Yoroi-san.