I've been thinking about this for a while now and it's honestly really difficult to choose just one, because there are so many I love. Group W, Kirkpatrick's Invaders, the Martian Cuirassiers, the Lexington Combat Group, the Blackhearts, the Knights of St. Cameron, Camacho's Caballeros, the Blue Star Irregulars, the Northwind Highlanders, Snord's Irregulars, Avanti's Angels, the Chaos Irregulars, Barret's Fusiliers, Khorsakov's Cossacks, Stalwart Support, Battle Magic, the Heart of Blake, the Fist of Mokal...they're all contenders for me. The sheer variety and creativity behind so many mercenary units is one of the things that drew me so far into Battletech.