The problem with answering this question is defining what "poor" and "country" are. According to the CIA World Factbook, the region with the lowest GDP (gross domestic product), measured as "purchasing power parity" is the island group of Tokelau, located in the South Pacific, with a GDP(ppp) of $1.5 million (US). However, Tokelau (a self governing protectorate of New Zealand) has a very small population (about 1,400 permanent residents). Based on per capita, reflecting the national wealth in terms of population, it may be a toss up between East Timor, formerly part of Indonesia, and the African nations of Malawi and Somalia, with a per capita GDP of $400 for East Timor, and $600 for the other two. East Timor's economy is still in a state of turmoil after the recent (and quite violent) cession from Indonesia. Malawi's economy is unfortunately common among many sub-Saharan African nations, wrought with corruption, lacking basic infrastructure, and especially troubling has one of the highest AIDS rates in the world. Somalia, similarly has been in a state of turmoil for decades, though due to civil war and factional in-fighting.
Taking into account other factors, such as national debt, literacy rates, employment rates, etc, in my opinion I would say however that Liberia is the poorest country. With a per capita wealth of only $700, an unemployment rate of of about 85% and about 80% of the population below the poverty line, its probably the best bet.