Colonel Shade has been with us since the beginning, her credentials speak for themselves, in combat she lays down lethal and inhumanly precise fire from extreme ranges.
Nagash is Shade's partner in crime, he's been with us just as long as she has, but an unfortunate incident confined him to this MEC suit, a situation which he has managed to turn into a huge boon for XCOM
Everybody needs a pretty face to be the front man, somebody with irresistible charm, Rommel is that man for XCOM. While arguably not the most capable soldier in the team, it's not hard to fall short of the astronomical bar set by Shade.
Surge, like Nagash, had been confined to a MEC suit after an ill-fated firefight that claimed the life of Captain Solitaire, but also like Nagash Surge has become one of XCOM's greatest assets in this form.
KC replaced Rath after she too was confined to a MEC suit after sustaining horrible injuries, while KC's contributions in combat are often few due to the sheer killing pace maintained by Surge, Nagash, and Shade, her ability to support the team with rockets is highly valued.
The man of the hour, Colonel "Quicksilver" Silver, our very own million dollar man. Silver has been extensively modified to push his abilities as a soldier as far as possible and then some, like Rommel there are better soldiers in the team than him, but Shade has set the "best in the squad" bar unreasonably high.