with that budget, there is a lot of different varieties of airsoft guns (none top of the line, but most pretty good). Really "best" when talking about an airsoft gun is something that simply cannot be done. Just do some research on airsplat, airsoftgi, shortyusa, and other airsoft websites with some key attributes in mind--
-get AT LEAST a full metal gearbox, get as most metal as you can on the rest of your gun also
-Read reviews to make sure this gun is reliable
-Get the highest fps possible(given)
-evaluate what kind of gun you want, for that price, it would be difficult to get a saw or be a support gunner, but you can decide whether to get a sniper (keeping in mind the cost of scopes), which if you get you should probably get a gas sniper, or to get an automatic (aeg). Getting an automatic is highly reccommended.
-there are two ways to buy automatics with the fairly low budget you have
1. buy the best possible stock gun for about $160-180 and at least get a better battery (this will usually up the fps and rof, as well as battery life), because stock batteries are honestly garbage
2. buy a gun for a lower price, making sure of the metal gearbox, and upgrading it. This can include a new precision barrel (at least 6.04mm or lower), new battery, new spring, new pistol head, and there are tons more things or send it in for a full upgrade, making sure it will make it you desired fps (350-400 is usually good)
I hope this helped, i have been doing this for a long time