AK-47 is the most practical because of the availability of parts, magazines and ammo. the AK-74 is the most fun to shoot, a bit more accurate, and a hot little round. Mags for the 74 aren't as easy to find in some areas, but tend to be much cheaper. Availability of ammo is a mail order affair in my area, unlike 7.62x39 which can be found anywhere due to the large numbers of SKS's out there.
I think I would get a 47 if I could only get one, and then work on getting the 74. I own both and love 'em both.