Well, I would suggest something that would be easy to upgrade, and has a variety of upgrade parts, and alot of guides such as DIY mods, along with how to disemble and reassemble the rifle. Therefore, I would suggest the JG bar-10, which is a clone of the vsr-10 and takes vsr10 parts, which are easy to find.
However, you need to be more specific. Whats your price range, what kind of rifle (longer or shorter), are you planning on upgrading, etc. etc.
Overall, I would suggest a rifle that has alot of information on it. Therefore, it will help you learn the ins and outs of the rifle, which can help give you the confidence you need to fix your rifle if things go bananas.
Bar-10 right about everything except for availibility I searched for 15 minutes to find one in stock before I found one at, brace yourself, Airsplat. I know I know shoot me. But If you can stand buying from them that would be your best bet.
Yeah, sorry about that. I meant availability of the upgrade parts which can be found on airsoft atlanta, and basically have everything that you would need. But the gun itself is typically out of stock everywhere. I know evike has a pre order for one, not sure when it comes in though. I know a guy on another forum who is selling a bar10, just in case you are having trouble finding one.