The fat-bodied one (that’s what the name means). Regarded by many as a form of Aylostera haagei, indeed originally described as Lobivia haagei var. crassa when discovered by Walter Rausch. Found to be distinctive enough to be given specific status by Mosti & Papini (2011).
The original description is here.
Plants of this species in the collection include:
Aylostera crassa HJ520
Aylostera crassa JO030
Aylostera crassa MN170
Aylostera crassa MN261
Aylostera crassa RH541/2
Aylostera crassa WR501