Well there are the traditional/obvious(to me) choices of Serrano, Jalapeno, Cayenne, and then there are the Generic Thai Reds, which seem invincible (super easy!), other than that you could grow one of the bird peppers of the species C. frutescens including the Tabasco, or a piquin/chiltepin of the species C. annuum for a little bit of a challenge (depending on your experience). Baccatums are fine and dandy as well if you want a "new" flavor, but I prefer to use them only as a powder.