Well, many answers...
If I had to buy one gun and I could only use it stock, and that would be the only gun I would ever have, it would either be an Angel or an Impulse vision. The Angel is hands down, the toughest electro out there. And it has cool factor, is accurate stock, and is pretty simple to understand and operate. In fact, if I had all the money in the world my field would be using angels as rentals cause they are so damn tough, and it is easy to pull the trigger. That is the main reason. With an electro, using the marker is as simple as clicking a mouse. Point and shoot. That is the only reason electros are better than mechanicals. No matter what you are doing, if you can touch the trigger, you can shoot. there is no technique involved like with, eg, a cocker. If I had to use CO2, though, I would have an Impulse. It is the same as an angel as far as how it works and it is much simpler, meaning less parts. It is also just as or faster than an angel.
As far as which gun gets the most out of upgrades, I would say any stack tube blow back. After market valves help the se markers with efficiency, and many other upgrades just better the quality of the marker and make it last longer. Autocockers are perfect stock except for the barrel as long as you don't futz with them. A friend of mine has an autococker from 95 that is all stock, barrel to that works perfectly. All he does is keep it clean and oiled.