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.
https://www.moxfield.com/decks/sR6Q8QgAuUyp7FfLG5cqPA
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.
I don’t know if he’s considered unpopular, but Chainer, Nightmare Adept is one of my all time favorite commanders, when he is leading rakdos dragons. Every card is a dragon, mentions dragons, or has dragons in the art. It is so slow and clunky, but I just love it. The main goal is to get Bladewing, The Risen onto the battlefield from the yard to cheat another dragon into play, to then bring Bladewing’s Thrall back from the dead. It’s stupid and not powerful, but it makes me feel warm and fuzzy inside.
I think Ulasht, the Hate Seed is the Commander I’ve used with the least decks. He’s like a Gruul Ghave, which gives you access to things like mana echoes and impact tremors, the fact that he can ping things also causes fun combos with cards like Scythe of the Wretched
I recently picked up a Karona, False God that looks like it could be a fun forced combat build.
I built Karona as an enchantress control deck. The goal was to pillow-fort myself, keep everything else off the board, and then MLD before a Solemnity/Decree of Silence combo. At that point, no one has anything and all they can do is hit each other with my commander.
I only played it twice, because it was deemed too mean.
I love my Gisela, Blade of Goldnight deck!
(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!