Modern Warefare 2 Multiplayer?

There are 4 types of cles with guns that you can use on MW2 multi player. ault Rifles - Sub Machine Guns - Light Machine Guns - Sniper Rifles - and 1 which is only a Riot shield. My question is: Which are the top 5 Mw2 guns to use and why? Is it because of damage? Speed? Ammunition?