It's all the Berries other than the Berries that already are an ingredient for another flavor. This took a while on my laggy iPod, but I got there in the end:
>Razz, Pomeg, Kelpsey, Qualot, Hondew, Grepa, Tamato, Cornn, Magost, Rabuta, Normel, Spelon, Watmel, Durin, Belue, Occa, Passho, Wacan, Rindo, Yache, Chople, Kebia, Shuca, Coba, Payapa, Babiri, Liechi, Chilan, Ganlon, Salac, Petaya, Apicot, Lansat, and Starf and the Berries that create Mild Poffins, as they do not already make Poffins flavors; they are the remainder.
Source: Common sence according to these threads:
http://bulbapedia.bulbagarden.net/wiki/Berries
http://bulbapedia.bulbagarden.net/wiki/Poffin
