I found shocktroopers to be the most useful, and snipers to be the least useful. Do snipers magically become better when they're a high level or something? Most enemies are behind sandbags, which means at best you're probably going to take 1/3 their health off. That's if your bullet hits. If the bullet always went where the crosshairs are snipers would automatically be better imo.
Scouts are really awesome if you can get right up next to a strong person like a lancer without dying. They're extremely situational though.