In theory, the Black Guardian (for example) is a far more powerful enemy than the Daleks ever could be. However, the Black Guardian is constrained by the equally powerful White Guardian & can't use his full power without encountering opposition that would effectively negate it. (See "The Key to Time".)
During the Time War, the Daleks came very close to defeating the Time Lords. That makes them the most powerful enemies we know of, in terms of power that has actually been used.