1. Mordin Solus
There is no character in the Mass Effect universe more interesting, more thought-provoking, more actually dangerous than Mordin Solus. One of the minds behind the perpetuation of the Genophage, the plague that swept the Krogan race, Mordin challenges the player's ability to like him. Everything in you should want to rip Mordin Solus apart limb from orange limb and beat him to death with them as he sang you adorable parodies of Broadway songs, and yet, inexplicably, you don't. Mordin has a code. Despite all he has done, he has a strict standard of ethics that will not budge. That will not move. When the chips are down in Mass Effect 3 and the Krogan race needs a saviour, it is Mordin who takes the steps necessary to save the day for everyone involved, making the ultimate sacrifice in the process. It's almost Shakespearian. He died undoing his life's work, realizing his mistake and taking action to right that mistake for the betterment of the world. That that, above all else, is why Mordin Solus is the ultimate Mass Effect squadmate.