Depends on the situation.
Vehicle killing: RPG, does more damage
Helicopter killing: M2 Gustav, faster and with any rocket launcher it's a one hit kill unless they have armor upgrade.
The At4 is fun to use, but does the least damage and is the hardest to control. (Getting heli kills with the AT4 is so fun to do though)
Mines I only use for launching friends over the enemy base with atv's.
Edit: And then there's the times where I'm an anti-infantry engineer, so I use an uzi with a 4 power, magnum ammo, and a gustav.