I have owned and shot umpteen varieties of AK-47's or their derivatives.
Your question asks "the best AK to buy?"
1. Probably a Valmet variation is one of the nicer made but these haven't been made and/or brought into the U.S. for years and they are very pricey and can be hard to find, although Gun Broker makes finding one easier, so this is probably not a practical rifle to buy unless you're a collector.
2. Pre-Ban foreign-made, arsenal-made (i.e. made in dedicated AK factory) AK. I prefer the Chinese stamped varieties with the slightly thicker 1.5 mm steel but the other varieties (i.e. Eastern Block countries and Egypt) are good and the milled varieties are good too. Be careful of buying a parts gun advertised as a pre-ban gun. Once again this is not so practical in terms of cost and ease of obtaining (except Gun Broker does help with finding one).
3. Post-Ban AK. These are from the 1990 and have "evil" features eliminated such as muzzle brake and pistol grip and have, in my opinion, the ugly but functional thumb hole stock. My favorite combination of ease and comfort of shooting and quality is the stamped Chinese MAK-90 (Modified AK). This has the ugly thumb hole stock but I like the LOP (length of pull) of that stock. Price and availability of these will be less than option #1 and #2 above but, most likely, you still will have to shop Gun Broker to find one.
4. Shop for a used Saiga or Arsenal. The last time I checked an original Saiga was pricey. A used arsenal might not be too bad.
5. I'll echo what other posters (Glashaus, Shell627, Inusuit, bulletslap, Triathaloncoach) have opined. The Arsenal brand (out of Las Vegas last I checked) is a good AK-47 and should be relatively easy to come by either ordering directly from Arsenal, ordering through a distributor, or buying off Gun Broker. Arsenal is not my favorite AK but it is very good. It is well-made and you can be sure you're getting a quality piece. About the only thing I don't like is the finish (painted).