I voted for Ohana, Nako, Minchi, and Sui.
Ohana was obvious, and in an one-vote only poll, she would have my vote.
Nako was a character that I always found pretty likable, and after her character focus episode, she really become much more well-rounded and memorable.
I wanted to limit it to three, but I couldn't choose between Minchi and Sui. Both characters I did not like at first, but they both became very strong, well-developed characters by the end of the anime, imo. What I liked about Minchi and Sui is that both of them were very practical-minded people who were willing to call other cast members out on their crap.
Sui was a bit more likable of the two overall, but the hilarious and useful "Minchi face" forced me to vote for Minchi as well, lol.