Rapid Fire (Allows you to make a double shot, even after Run and Gun - take this if you use shotguns).Flush (Knocks an enemy out of cover so your allies can decimate it).Sergeant: Lightning Reflexes (enemies rarely go on overwatch, but it can be particularly useful if the enemies do actually use it).Corporal: Tactical Sense (really neither of these abilities are good - if you're surrounded by enemies you're likely dead anyway, but it's better than unnecessary critical chance).Squaddie: Run and Gun (allows you to shoot after moving twice).For weapons, it will depend on your playstyle but I always stick with Assault Rifles. Accordingly, it's best to get defensive abilities to make sure your Assault can actually be effective instead of dead. I think the class is incredibly weak as it needs to get very close to do damage, which means it will often be surrounded by enemies. Shotguns do more damage close up and have a huge aim penalty far away. This class can use assault rifles and shotguns. Colonel: Double Tap (allows you to fire twice in a turn, which is simply too good).Captain: Opportunist (removes the aim penalty when on overwatch, which works with squadsight, making your overwatch a huge threat to the enemies).Lieutenant: Disabling Shot (can be useful if you're up against an enemy you don't think you can kill in one turn).Sergeant: Damn Good Ground (confers extra Aim if you're higher up).Corporal: Squad Sight (this is the best ability in the game - NEVER take the other option).

Squaddie: Headshot (increases critical strike chance).

You'll want 1-2 of them on every single mission. This is by far the best class in the game.

Mayhem (Rockets and grenades do more damage with better weapons - +3 with plasma, making this an great ability).

Rocketeer (Allows an extra rocket per battle which is amazing).

Colonel: Both abilities are good so you can pick whichever you want.

Major: Will to Survive (the only option).

Captain: Danger Zone (increases the radius of your explosives and suppression - some don't like this as it can have unintended drawbacks, but I think it's worthwhile overall).

Lieutenant: Heat Ammo (50% extra damage against the toughest enemies in the game - an easy choice).

Sergeant: Shredder Rocket (this gives you more rockets and allows you to take down bigger enemies faster).

Corporal: Bullet Swarm (allows you to attack twice in a turn - extremely useful).

Squaddie: Rocket (the Heavy's bread and butter).

For abilities, I recommend you take the following: Their accuracy isn't extremely high, but they can fire a rocket launcher, giving them far more damage.

Xcom enemy within assault build