Zaeed, Legion, and Grunt
Zaeed is arguably the most versatile squad member in the game and you get him early on with his loyalty mission immediately available which is a huge plus. He gets concussive shot to take down barriers, disruptor ammo for shields, and inferno grenade for armor. Sniper rifles and assault rifles are also the best two weapons in the game when it comes to medium-long range damage output and his class power when fully upgraded can give him up to +50% damage which is tied with thane for the most amongst your squadmates. He's essentially a better version of garrus for all intensive purposes. (same weapons but better powers). Until I get legion I take him on every single mission as he combines tremendous damage output with excellent versatility.
Legion is the arguably most powerful squad mate in the game. With two geth shield upgrades he has 200 health and a whopping 525 shield (compared to 200 health and 250 shield for most other characters) and with the geth shield boost power his durability is on par with a fully upgraded grunt while his widow sniper rifle does an absurd amount of damage. His two powers, AI hacking and combat drone, are also quite useful. It that wan't enough he also gets to use assault rifles for close range combat. Basically once you get him there's no real excuse not to take him on every mission after that.
Grunt is on par with legion when it comes to durability with base 450 health and 300 armor (when fully upgraded) with the additional benefit of rapid health regeneration as well as his fortification ability which mirrors the geth shield boost power. While legion is the best at medium-long range grunt is extremely effective at close range with his claymore shotgun and krogan charge. His powers are also tremendously useful with concussive shot tearing down barriers and blasting enemies to the ground while incendiary ammo, when fully upgraded to squad incendiary ammo, provides a huge boost to the whole squad's damage output.
I should also mention garrus as he's similar to zaeed in that he has versatile powers (AP ammo for armor, overload for shields, concussive shot for barriers) but has inferior damage output and inferior powers in that disrutpor ammo is more useful than AP ammo and inferno grenade is more useful than overload. Garrus only gets +25% weapon damage while zaeed gets up to +50%. When I acquire garrus I usually take him on a few missions but stop using him (except for his loyalty mission) when I acquire grunt and later legion.