The list is almost endless... in a tank that size you could go for a colony of the smaller species, or a trio of the larger ones. Apistogramma borellii - a personal favourite and a lovely peaceful fish, should thrive and are little gems, they're also often available which can be a real problem with Dwarf Cichlids - lack of availability. All the species mentioned by Darrel would be good choices, and I'd also add A.trifasciata, A.hoignei, or A.viejita (which may not actually be A.viejita, but often sold as it). To be honest there are only a handful of Apistogrammas which are really aggressive (A.sp. "Steel Blue") and/or delicate (A.elizabethae).
Then there are fish such as Dicrossus filamentosa (or D.maculatus if you can find it), Nannacara anomala (females are a bit feisty when guarding young), and the Ram M.ramrezi.
If you're in China there will be a lot of man made fish available in the local tropical fish markets, especially for the Rams and A.agasizii, I don't know why but the Chinese market seems to particularly like mutated fish of this kind. My advice would be to avoid them like the plague, they are (like fancy Goldfish) usually sickly and short lived. The wild type may not look so attractive as juveniles, but the adults are far more attractive and hardy too, plus you're much more likely to have success in breeding them (one of the main attractions of Dwarf Cichlids).