Here's my findings on some of the weapons;
1) Adas Assault Rifle - Ties the Particle Rifle for the best squadmate weapon in the game. Why? 2x damage modifier against shields (which there is LOTS of in single player), "area of effect" blast radius which can hit multiple enemies in close proximity. Squadmates also actually will fire the gun continuously at its maximum RoF (something they dont do with a lot of weapons). Negatives = Massive screen shake when close to squadmates, not recommended when using scoped weapons, deals only HALF damage vs armor making it weak against Brutes and Banshees.
Weapon Mods = Extended Barrel + Extended Clip + Inferno, Warp, or Armor Piercing ammo (NOTE: The AP mod does NOT work on this gun and has no extra effect towards armor - the ammo powers however will increase the damage towards armor)
2) Blood Pack SMG - Easily the best squadmate SMG, and for all the squadmates that cant use Shotguns or Assault Rifles, this should be the gun to use. Highest damage per bullet for all the SMGs and the highest DPS for all SMGs as well. Your squadmates are very accurate with this gun as well. It also applies ammo effects very well, so any Squad Ammo power works good.
Weapon Mods = High Velocity or High Caliber Barrels + Extended Mags + AP, Warp, or Inferno ammo - if you use AP ammo, switch to High Caliber Barrel
3) Indra Sniper Rifle = Fairly good on squadmates because of the high rate of fire and the fact that squadmates will generally fire most of its clip each time. HOWEVER, that said the problem is virtually every squadmate that can use Sniper Rifles can use Assault Rifles, and unforunately none of the Sniper Rifles beat out the Particle Rifle or Adas Anti Synthetic.
Weapon Mods = AP mod + Extended Barrel + ANY ammo power(they all work really well.)
4) Cerberus Harrier Assault Rifle - Theoretically speaking this gun SHOULD be the best squadmate weapon, realistically however it falls short of that lofty expectation. The problem is your squadmates wont fire the entire clip on full auto each burst like they do with the Adas and Particle Rifle. It still does decent damage however, and its a bit "safer" for squadmates because it doesnt require the exposure time that the Particle Rifle and Adas do. It also can pierce cover and doesnt shake your screen unlike the Adas. Again, the DPS of this weapon means it SHOULD be the best weapon for squadmates in the game, but they just dont shoot it enough to exploit its insane potential.
Weapon Mods = AP mod + Extended Barrel + Any ammo power
---Rest of the weapons are garbage on squadmates ---
Krysae - They only fire 1 shot instead of all 3 (most of the time), and are very poor at aiming with it, so it has very pathetic damage on squadmates.
Reegar - the short range "flame thrower" nature of the weapon makes your squadmates stupid with this gun... shotguns for squadmates should always be the Geth Plasma, Graal, or Claymore.
Geth SMG - Squadmates dont fire continuously enough with the gun to make it worthwhile, and the damage per bullet is so pathetically low that squadmates are better off just staying in cover so you have their powers.
*TL;DR* - Adas and Particle Rifle are easily the best squadmate weapons in the game... easily. The Blood Pack SMG makes the SMG weilding squadmates decent damage dealers now, especially with ammo powers included.