I'm far from an expert, and don't even play one on tv, but I'll have a go at it. Menghai is a county in Xishuangbanna, Yunnan. Many producers seem to have attached "Menghai" to their name, no doubt due to the fame of one factory in particular: Menghai Tea Factory, owner of the Dayi brand name. The Menghai and Kunming tea factories were among the first to produce cooked puerhs, so it's not surprising that they're two of the big boys in the world of shu. Xiaguan seems to specialize in tuos, but I wouldn't want to paint with too broad a brush. These three factories, as mentioned earlier, are the old guard who have "earned" a number (2,1, and 3 respectively), along with Haiwan  and Feng Qing/Lancang, who, as far as I can tell, are fighting it out over the rights to the number "4".