They're all great, but some require more... support than others.
Rangers can do some serious damage, but you'll need a good, upgrade Revolver, with max Death by Revolver buff and Sharp Eye Potions for max crit damage. You'll do fine job on your own with your kick skills, Windmill/Topspin, Mach Kick/Needle Sobat, and Air Raid/Sonic Spike, but you won't be dealing any outstanding damage.
Spitfires require the correct build to do well in PvE. You will need max Piercing Bullet, Blazing Bullet, and Flash Bang to maximize damage. Flash Bang is needed for its electrocution effect for extra damage. Blazing Bullet explodes once upon hitting the enemy, and once when hitting the ground, so you'll need Piercing Bullet to get both explosions for maximum damage.
Launchers have the best crowd control skills, with a piercing Laser Rifle and large range Flamethrowers, but they also have the largest MP issues. Even if you go into a dungeon on King's Road with full MP, expect to use it all and still not have killed the boss. It's a very consistent class through and through -- consistent in damage, consistent in mob control, and consistent in devouring your MP.
Mechanics have a constant flow of AoEs, either by using Land Runners and Time Bombs, or by using your rapid-fire turret, Corona. You'll have to do your best to play keep-away though, because you're a bit fragile from your Cloth Mastery.