I studied up on this a couple of months ago and consensus was the best route to go with was the Ultimak gas tube replacement.
http://www.ultimak.com/
There are people that like all different options and have had success with them, but what I learned and I'm sure someone will correct me, was the following:
-Dust cover mounts are a waste. There was only one person that claimed they hadn't had a problem with it moving somewhat. Also to clean the weapon you have to then remove the dust cover mount and red dot scope. Could affect zeroing and other functions.
-Side mounts are fine, but the ones I saw were mostly completed with Kobra optics. Some people have had great success with their Kobra's, others not so much. Also it is almost impossible to co-witness your iron sights with the side rail mount and many examples seem to have a hard time getting the actual scope centered above the gun. Not all, but most. Some people also complained about the Kobra optics, but overall many were content with them.
-Lastly the Ultimak rail replacement in a scout squad set up. This had just about unanimous support from those that had tried it. It SOLIDLY locks in your rail, it doesn't move a bit. It centers the red dot, and if you go with an Aimpoint like I did, with a low profile mounting ring, you can co-witness your iron sights as well. Installation was simple, zeroing the red dot was a piece of cake, and the function has been awesome. One thing I worried about was having the sight forward mounted, but surprisingly, and nobody mentioned this, the AK having a short stock to begin with, works great with the forward mounted optic because it is then a perfect distance from your eye.
So to sum up. I got the Ultimak, with a low profile 30 mm scope ring, and an Aimpoint CompML3 red dot. I went with the CompML3 because it was cheaper than the Aimpoint Micro's, has a rubber protective sleeve, has 9 different brightness settings, Aimpoint quality and battery life.
Like I said, lots of different options, but I think the Ultimak is the way to go.