Strongest of all genlocks encountered so far is the Genlock Forge Master. He is based fairly deep within the Dead Trenches, in his own little alcove (which makes him an optional boss). He is a reasonably strong opponent, not helped by the fact he is surrounded by many darkspawn (Hurlocks, Genlocks [mostly ranged Genlock Archers], and one Hurlock Alpha). It is advised that these are drawn out one by one first, before engaging the Forge Master himself. He will use a bow (and the Arrow of Slaying talent) until engaged in close combat, where he will use a Maul. His hitpoints are substantial, and fighting him one-on-one can be tricky if he still has allies available to tie up your party or chip away at your health.
See: Dragon Age: Origins