Crenicichla regani is the most common dwarf, said to max at 6" but I've never personally seen one over 4". Can be kept with larger or high bodied tetras even as well as dwarf cichlids like rams, apistos, ect.
Crenicichla notophthalmus is probably the prettiest though much harder to find. A strict blackwater species, found with altum angels and heckel's discus.
Crenicichla wallacii are becoming more common now, being found in the Rio Atapabo and Rio Orinico. I have mine with rummynose tetras and dwarf jewels. A little scrappy towards each other, but mostly minor nips and a few split fins on the females.
Crenicichla compressiceps is by far the easist to get, but they don't act like the above three species. These fish hate each other with a passion. Despite maxing at 3-4 inches, one gent had them kill their way down to a pair even in a 300 gallon tank. I've had them kill Jack Dempsey's over twice their size.
There are others, but they are rarely if ever seen (orinico dwarves, xingu dwaves, citrus dwarves, ect ect).