I've been thinking about this question recently too. As far as I can gather so far, Qantas is the easiest FFP to obtain Emerald status requiring fewest flights/miles (but has that 4 QF requirement).

There appears to be a number of mentions that American Airlines is a good option for infrequent flyers.

I am not sure about other factors such as reward levels or other benefits or privileges that may play a significant role in people's decision on the best program. Maybe the more experienced minds here can shed some light.