Really general topic, and there are tons of airsoft brands around there.
Chinese brands (JG, Snow Wolf, HFC, A&K etc.) are generally your cheaper end guns. Plastic is okay, internal wise they skimp out, and they like to come with high FPS. What most advanced airsofters will do though, is they'll take these guns and upgrade them internally to be good performers.
Hong Kong brands though (e.g. King Arms, G&P, ARES) tend to have much better quality overall than the aforementioned chinese brands. If you're looking for something somewhat affordable, these are excellent choices.
Taiwanese companies produce good stuff. WE Tech makes great GBBs; G&G, KWA, ICS, VFC all make excellent AEGs. These aren't always cheap though, but they're superior in stock form.
Japenese companies (Marui pretty much) produce high performing, reliable stuff. They created the AEG in essence, and are still reinventing it. Marui GBB pistols are also pretty much the best things out there. The only drawback is that these are all plastic (The new AEG line comes in metal though). The plastic is very good quality, just, well, that still doesn't make it metal