If you don't want a low-end marker, stock cockers seem to be the most popular choice cost-effective wise. I personally bought a Piranha STS when I started out and beat the living daylights out of the thing for about 1,5 years. It did a great job with a new barrel and delrin bolt. Though it's low-end, it's cheaper than a cocker and they say Piranhas work very well compared other paintguns in the same price-category, like Tippmanns, Spyders etc.
Try out different ones if you can. Your choice will be easier.