Mine is definitely Hua Tuo, Honored Physician. I love decks with consistent value engines, and he's an interesting commander with less than 150 decks on scryfall.
Note: I tend to build budget decks because I am a poor student with a lot of debt, though the only expensive upgrades I can think of would be cards like Apex Devastator .
Sideboard is filled with cards that I haven previously had in the deck. Felt that pipers like howlpack piper and Champion of Rhonas are clunky because I generally want to be casting creatures from the top of my deck, not from my hand.
(Not sure how unpopular she is - > rank 500 on EDHRec at least)
She's very powerful for a 7CMC boros commander - I built her with a lot of group slug effects, where she breaks parity. With her damage doubling she can go off like a bomb!