Gymnocalycium baldianum

Gymnocalycium genus is named for naked flower tube, clearly seen in the picture. Buds are very characteristic for the genus, and once you see those white scales there is no doubt what it is.
Gymnocalycium  is a large genus of compact plants, what  makes it very popular. Plants are generally easy to grow and flower. Flowers are generally white, but there are species with red and yellow blooms.
Gymnocalycium baldianum is probably the most common and easy to grow plant within the genus, well known for it's bright red flowers. There are newly discovered wild populations of this species with pink or white flowers, but such plants are rarely seen in culture.