Best squad build?

There really is no best squad build. Each character has advantages and disadvantages in battle, so experiment with them and choose them based on what skills you like.

These are my recommendations, and the reason for it:

Overload - you need a weapon that takes down shields and biotic barriers, and this is the best one for it. Power this up to maximum.
Warp, Carnage, Incinerate, or Cryo Blast - all weaken or destroy armor. Power one of them up.
Since most enemies in the game have shields / barriers and/or armor, these will be among the most used powers.
Sabotage (formerly A. I. Hacking) - Unfortunately, you don't encounter Geth nearly as much as the previous two games, but when you do, this power makes you incredibly powerful. Also works on turrets, which are incredibly powerful and can clear the room when hacked. Put at least a few points into this.
Combat Drone - Not to be confused with the nearly-useless Defense drone, these are an excellent way to distract enemies or flush them out of cover. Fully powered they fire devastating rockets. Put a few points into this.
Tactical Cloak - So effective, it borders on cheating. If enemies are shooting you, use this and they'll stop attacking. If you power it up enough, it gives you plenty of time to escape sticky situations and run to safety. Power this to the max!
You should also get a special ammo type - armor piercing, incendiary, warp, or cryo. Don't spend points beyond the first two or three ranks until you've got everything else beefed up, as the rewards are rather slight compared to other powers.

These are my least favorite powers:
Defense drone - Unlike the Combat drone, this never stays far from you. When facing melee enemies, like husks, or if you play as a vanguard and specialize in close-quarters combat, it can be useful, but if you tend to keep your fights at a distance, you'd be better off with something else.
Disruptor ammo isn't very useful, since you mostly fight organic enemies, and overload is a much better way of taking down shields.
Turrets are also not very good (in my opinion) - even fully powered up, they aren't as good as the ones the enemies get, they're hard to place with precision, and enemies take them out pretty quickly.
Fortification - One of the worst powers in the game. Slows down cool down times to give you a melee bonus. Not worth it, in my opinion.
Tech armor / defense matrix - Like the above, they slow down cool down times, though the added defense and ability to detonate them makes them useful in certain situations. I use powers frequently, so I avoid anything that slows that down. If you rely more on your guns, these might be of more benefit to you.

I was a Vanguard. I had Decoy as my extra power and only Cryo Ammo. With Kaiden maxed out on Cryo Ammo, Reave. Barrier and his class specialty, plus Tali with maxed out Sabotage, Energy Drain, Combat Drone and her class specialty - we wuz like a hot knife slicing through butter. Reave/Energy Drain/Cryo is a trifecta of destruction. Made Brutes much easier to deal with. Those two were so good I could hang back and snipe everyone, while they led the charge. Also, max out your team's gun levels to V down at Cortez's station.
With Kaiden, you get a biotic who can carry an assault rifle and Tali uses a shotgun. SMGs suck.

I almost forgot, since you can't get Kaidan and Tali until later missions, Edi and Liara are pretty good.