Its a close one between Ezlo and Midna, and heres the reasoning:
Ezlo
Ezlo was just a witty, humorous character that provided tips on occasion in a quirky way. He has backstory, (yes I know, not nearly as much as Midna)and looks ridiculous.
Midna
Midna was the most helpfull out of all the sidekicks that Link has ever had, provided jokes from time to time, and had a LOT of backstory.
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In the end, I think Ezlo comes out on top....it's the wit that does it for me.