What makes a good Cocker? Ah, the golden question of gun tech. I think we've all hit this a zillion times, but here goes my list:
1. Good valve. Tornado, Rocket, RAT 3, or MacDev RED. Why? Efficiency, low pressure, and quietness. Must be sprung correctly to work.
2. Proper hammer to go with the valve and springs.
3. Good LPR. Jackhammer 2 (what I use) or FGP. Sonics are good but hard to know how to adjust properly. Palmers are notoriously reliable but must be set, and quite honestly by the time I have a Cocker put together, I want it running THAT DAY, NOT A FEW THOUSAND SHOTS LATER. Simply put, if this part of your gun isn't working, the gun won't cycle correctly. Period.
4. Good 3way. I like Shocktech Bombs (short pull), ANS (can pull the shaft out straight through the front for lubing without taking the pneumatics off to get to it), and Bullets best. KEEP THEM LUBED OR YOU'LL BE SORRY!
5. Well bored out body. No, I don't mean the milling, but a gigantic air chamber up front. HUGE. You want it. If you don't have it, you'll wish you did. $400 milling and anodizing is in my opinion thrown away if you don't have enough air room up front to get 275-250psi operating pressure without a volumizer.
6. Good trigger plate and frame with set screws in every possible position. Rather than having to fumble around with a zillion sets of springs and bend plates and try idiodic manipulations that could more likely ruin your trigger settings, springs, and even frame, just get one with proper set screw placements so you can do the work yourself by simply adjusting screws and then MAYBE changing to a slightly different weight of sear and trigger return spring. DO NOT CHANGE SPRINGS UNTIL YOU HAVE THE OTHER SETTINGS RIGHT.