The African giant black millipede, A. gigas, and the largest millipede regularly kept. Adults exceed a foot and are about as harmless as a large invertebrate gets.
Detritivores, slow, docile, long lived, and genuinely pleasant animals that will sit in an open hand without incident. They need deep substrate rich in decaying hardwood and leaf litter, which is the actual husbandry: you are cultivating a substrate, not feeding a pet.
United States import of foreign millipedes has been effectively closed for years on agricultural pest grounds, which is why this species costs substantially more in North America than in Europe and why US supply depends entirely on domestic captive breeding. That regulatory gap is the largest single price differential we track for any animal.