The more technical term is lower receiver extension; however "buffer tube" seems to be the more common term for it. Either way, I am talking about the aluminum tube that holds the buffer and spring on the inside and supports the stock on the outside.
As most people want a collapsible stock, most of the time the choice for them comes down to commercial or milspec. It is commonly accepted that milspec is stronger, but I have yet to find a study to confirm that. Some argue that the slightly thicker commercial tube in in fact stronger. I am not willing to destroy parts enough to test it but from my research I think that while thicker is usually stronger, the issue with commercial tubes comes from the threads and how they fit into the lower. They should be the same, but they are not always. Milspec threads make for a stronger fit and, as the connection to the receiver is more critical to the weapon's overall strength - I stick with milspec.
You can pick up a DPMS or UTG milspec buffer tube from Midway for under $20, or Amazon for just about $20.
PWS Enhanced Buffer Tube/Reciever Extension compared with a DPMS milspec tube
Another advantage to milspec is almost any after market stock will fit on it, though the commercial ones are loose and rattle. If you buy a commercial tube, a milspec stock is rarely able to slide onto it. Don't beat your stock onto the tube if you find it is super tight. It is likely you got a commercial tube and milspec stock.
So who do you buy the tube from? Well, there are not that many places that actually make their own so generally I advise going with the cheapest milspec available, which I usually find is DPMS or Double Star from Brownells.
However if money is not an object, the best you can get is the PWS Enhanced buffer tube which runs about $100.
But wait! There is more!
Magpul UBR Stock
You do have a couple of other options. Firstly, magpul makes a stock called the UBR (Utility/Battle Rifle), which Magpul says it is the most solid collapsing stock around and I have to agree with them. It is heavy to be sure, but the weight is at the back and I love mine. It is rock solid. But it is pricey at over $250 most places. It comes with its own tube though, which means while you don't have to pay extra for a tube, you can't change out your stock as easily.
The other option is to go with an older A2 rifle length tube and then put on a fixed stock like the old style or better yet, a magul PRS stock.
In summary, if you are on a budget get the DPMS milspec tube for under $30 (at Brownells with an account). If you want the best tube get the PWS and if you want the most solid collapsible stock get the magpul UBR.