You be a sniper. Then pick any combination of 2 of the other 3 snipers. Preferably Legion since he has the upgraded sniper rifle, so drop Thane or Garrus. At this point you can just afk because their weapons have no ammo and snipers kill anything very fast.
Any other combination is just so inferior it's dumb. In fact the game's already mediocre gameplay is reduced even further due to this combination.
Horn-swaggle, I can kill entire rooms by myself faster than this noobsauce combination, and all I need is Heavy Charge, Champion, Inferno Ammo, and the Krogun. And shit, Legion with the Widow does a lot less damage than Legion with the Inscisor. You have no idea what you're talking about - Garrus and Legion shouldn't be in the running regardless, because Thane and Zaeed both get 50% to weapon damage, which makes them much more efficient users of the Inscisor than anyone else.
Come back when you know how to break a game.
To be on-topic:
The fact of the matter is that the most efficient team will vary according to the mission that you have to complete. More, certain Shepard builds - like Vanguards - are so efficient that teammates aren't really there for any reason except to lay suppressing fire and maybe hit the enemy with staggering powers through their shields.
Most efficient in terms of DPS, though? Team dynamic?
Take Sentinel Shepard and give them both Heavy Warp and the class specialization that enhances power cooldown the most. Take Samara and Jack, give them Pull Field, and then their class specializations that enhances biotic cooldown the most. Knock down one enemy's shields. Samara pulls. Shepard detonates with Heavy Warp. This will knock down the shields of almost everyone around them. Then Jack pulls. Then Shepard's cooldown is already done - detonate with Heavy Warp. Bodies flying everywhere.
Generally, though, any team with Miranda will excel - her class specialization that adds 15% to party weapon damage can make Shepard even more terrifying, which is always the best benefit to have.