Dwarfs. Most of the genus tops out around two inches, some under that, and they carry a distinctive dark heart-shaped marking on the abdomen that is the easiest field ID in the hobby.
Small does not mean simple. They are quick, they burrow, and a two-inch adult in a ten-gallon tank is a two-inch adult you will never see again. Keep them in small enclosures on purpose. C. elegans is the common entry, affordable and forgiving.
Dwarf species occupy a strange market position. They cost less to ship, less to feed, and less to house, so prices stay low, but the rarer localities out of Peru and Venezuela have thin supply and can spike hard when a single importer stops bringing them in. Watch listing counts here more than prices.