Broken/The Helicopter Sniping Trick
This trick only applies to Vice City, San Andreas, Liberty City Stories, and Vice City Stories. There is a way to destroy a helicopter in one shot with a sniper rifle. When you have an opportunity, if you have a sniper rifle equipped, zoom in on the helicopter and aim so your cross hair is on the spot where the blades meet the top of the helicopter and fire. The helicopter should lose control and begin descending until it crashes. It's quite easy to do in all games but you must be more precise with your shot in San Andreas.
The helicopters in GTA IV have increased armor, making them nearly immune to small arms (except RPGs). Repeatedly firing at the hull will eventually cause it to smoke, but this can be difficult. However, the cockpit of a Maverick has large glass panels and the pilot is clearly visible. If you can shoot the pilot with a sniper rifle, the helicopter will be sent into a death spin and crash. In addition, killing the two cops who are providing cover fire from the cabin will cause the helicopter to retreat. Annihilators are immune to all weapons except RPGs.