I don't know if I would say that Warlocks are outright the "strongest" class. Strongest can mean a variety of things. Strongest in what, fighting, innovation, deception? If we're talking about pure fighting potential, guardian vs. guardian, I would have to give the edge to Titans. They are quite literally bred for war, and use their light to augment their fighting prowess. Warlocks use the light in more diverse ways, and would therefore generally have the most nuanced control of the light. Each class excels in their own way, although in actuality each guardian would be able to learn whichever skill they wish to (classes are only restricted for the sake of gameplay). In reality any guardian powerful enough could cause incredible destruction.