Well,as some people know, I've got an assload of pistols, ranging from low cost WE and KJW, TM, KSC, WA and even a couple of full customs. Some look better than others, but 2 of them really stand out to me.
First if my KSC Glock 17. This thing has been my workhorse pistol for years now. It's been reliable, stupidly accurate, and just a pleasure to shoot. I'll echo Stalker's assessment of the pistol for its angle / ease of use. I have a respect for the Glock I never really had before because of this gun.
Secondly, the newer addition to my collection that is rapidly turning into my favorite gun - the Tm FN 5-7. I originally bought it as a collection piece to have as the mate to my P90. I quickly discovered this pistole is actually quite amazing. The range and accuracy are just stupid they're so good. And the large mag size assures you have plenty of gas to get off at least 2 full mags worth of shots at full power.
However, what amazed me most was the gun's performance in cold weather. At -10º C - a temperature that will render most GBB pistols inoperable - this thing still shoots hard and reliably, getting a full 2 mags worth of shots out of 1 gas fill. Even after having found a mag that I had lost in the snow a couple of hours earlier, I put it in the gun, and got off 2 full mags worth of shots. To me, that's nothing short of amazing. And the fact that the real gun is completely plastic means you don't need to upgrade parts to make it more realistic.
That being said, if you go down this route, I would recommend right away replacing the shitty aluminum outer barrel with an aftermarket steel replacement (about $15). Due to its design, the aluminum barrel will break under the stress of propane.
If money is an issue, then my top recommendation is the KJW Beretta M9. I had one for 3 years, shot mag after mag out of it, and the only issue I ever encountered was the plastic mag catch wearing out. Otherwise, the gun works like a charm. I got a TM-based UNC custom M9 a while back, so gave my old KJ M9 to a friend for his bday, and as far as I know, it's still working great, depite him being utterly cursed with pistols.