That's just not possible in the Internet age. There's no information assymetry to facilitate the existence of such camera. During the pandemic I've gotten into the camera collecting rabbit hole, and what I have found is that a camera online reputation is always remarkably accurate. There are no sleepers.
Eh... you're not really accounting for the laziness of your average "new to analog" shooter though. If I had a nickel for every post on Reddit analog groups to the effect of "I'm new to film, looking for a good 35mm camera to take pictures of my friends, and sports, and wildlife, and landscapes, and the night sky, and close-ups, what do you recommend?" And then another nickel for every comment underneath from people who have shot film for 4 months, invariably recommending the K1000, AE-1P, and Contax T3... you get the idea.
In that spirit if you define "sleeper camera" as a camera that is very good for popular types of photography and has a disproportionately small user base these days, and/or disproportionately low price... I'd say Canon EOS cameras, anything from the Minolta SR-T lineup, anything by Ricoh, about 2/3s of the manual SLRs produced by Pentax, Olympus, or Canon (anything that isn't a K1000, AE-1, F-1, A-1, or OM-2n)... the list goes on.
I'd also submit for your consideration the Zeiss Ikoflex TLRs. They get no love compared to the Rolleis and Yashicas of the world, but mine has a really excellent Tessar lens and produces beautiful and crisp negatives.