Systema is the best. Police and Military use them for training. However, they range from $1,200 - $2000. Also, they have all their own components which are expensive along with it's own brand of mags (though there are non-Systema brand mags that work).
Tokyo Marui is next in line for the relative title of "BEST." TM is one of the originators and innovators. Their guns are highly detailed and extremely smooth performance. And for anyone who whines about them not being metal and shooting low: They make metal guns and FPS doesn't = Awesome.
KWA is ranked 3rd for me. Their guns are very high performance and KWA has the longest warranty (45 Days) which shows they actually back up their products. Their rifles are becoming renown for being durable and high performing and relatively cheaper than other high-end brands.
Your question is too general.
If you were to ask for a good brand if you're starting airsoft, I would say Crossman. Get yourself a spring pistol and a cheapy electric gun, go out into the woods with your friends and try them out (with proper eye protection.)
If you were to ask for a good brand if you've been into airsoft for a while and find it exciting but don't want to spend too much money in case you lose interest, go for Echo 1, G&G, CMYA or JG.