Its really hard to quantify "best" in an AK.
A few general points you should know: They're not as modular as ARs. If you think you want an underfolder, you better order it that way. Side folders can be added to fixed stock guns, but in most cases they'll either require you to cut the tang (not a big deal if you never plan on going back to a fixed stock) or stick out a little funny.
Since AKs are made by a lot of different builders, countries and companies, there are a lot of variants out there.
The biggest thing you need to decide is caliber. The traditional AK-47 caliber is 7.62x39. Its an intermediate cartridge that a lot of people compare to the 30-30. It has more penetration than most intermediate calibers, and its still fairly cheap to buy. I like it.
The other Russian caliber is 5.45x39. This is the Russian AK-74 caliber. Its faster and lighter. Recoil on the '74s is softer as well. The only slight concern is that no US company is producing it (as far as I know) and so if ammo import is ever banned, the '74s might see the ammo supply dry up fast. But for now its pretty cheap to stock up on, so that's the counter argument for it. Plus I'm sure if there was a demand for domesticly made 5.45x39 for whatever reason, someone would step up and do so. I'ts just pointless right now because its cheaper to import it.
The last "big" option is the traditional AR-15 caliber 5.56x45. Nothing wrong with an AK in this chambering, but magazines are a little more expensive/harder to come by.
If you have some general firearms/tool knowledge, I highly recommend buying a Saiga. They're Russian AKs made for import. With a few parts, you can restore them to their proper AK form. Here's a website about the conversion:
http://www.cross-conn.com/Saiga_Conversion/
You can still get them with that style handguard, but if I was doing another conversion I'd start with this:
http://centerfiresystems.com/SAI-762IZ332.aspx so all you have to do is add a muzzle device, stock, fcg and pistol grip.
The AMD-65 that others mentioned are a darn good deal right now.
http://centerfiresystems.com/AKAGUN-AMD.aspx
The Romanian WASR-10 is commonly considered your "entry level" AK. They're not badly priced, and the people I know who have one are very fond of it.
http://centerfiresystems.com/AKAGUN20.aspx
Here's an underfolder option:
http://centerfiresystems.com/AKAGUNUF.aspx
Fixed stock '74 style:
http://centerfiresystems.com/akagun-bul-wd.aspx
Folding stock '74:
http://www.atlanticfirearms.com/index.p ... Itemid=763
I'm sure I left some stuff out that I'll remember later, but just wanted to throw up some more options as well. I didn't include Arsenals because someone already mentioned them, not because I have anything against them. They're nice guns.
Good luck on your search!