The one you can afford
Nerf gun
The best airsoft gun you could get for $150 would probably be an Aftermath gun, but you could get a JG M4 or M16 for under $150
If you are just starting airsoft i would highly recommend an aeg, (airsoft electric gun) but if you are pro and experienced, a trained camouflaged sniper is the best, an invisible predator. However it does depend on the user of the gun. If you are asking performance wise then g&g makes the best airsoft guns on the market. (currently in the US)
I say twin ozez are the best cheap airsoft guns
dont get that gun walmart has the best beginer guns
www.shortyusa.com
Definitely Nerf.
it depends on what cheap is to you
The best way - go to near airsoft workshop, give them money and tell what do you want. If you decide to modify airsoft gun by yourself - be warned, this may ruin your gun and even be dangerous to your health.
"Systema" is by far the best airsoft gun brand out there.
An Airsoft gun
A pump airsoft gun is a gun that has to be pumped every time before shooting. Ex. Airsoft shotgun