10
Personally I like the Sentinel in 2/3 the best but in 1 they don't really have the specific niche purpose that they seem to in the sequels, since all dual classes just get half of the powers from each.
ME1 I just go full Engineer and select AR training.
It's always AR training.
Soldier's a ton of fun. Everyone craps on it because it's really plain in ME1, but in ME2 and ME3, they REALLY overhauled the character. In ME2, adrenaline rush is just broken, especially with stuff like the Revenant. (oh, and you get the Revenant on your first playthrough.)
ME3 is ridiculous though. You get all the ammo powers, and you can chain ammo power explosions for everything but warp ammo with concussive shot. It's kind of cheaty though, because if you take yourself, Garrus, and Ashley and equip everyone with Typhoon V-X, everything dies in seconds.