Before I start, I'd like to note that I am counting both pre-and post-fusion feats as valid. Whether or not you agree is up to you, but I'd like to avoid any arguments on the subject on this thread. That being said, let's get started:
Unalaq is the Chief of the Water Tribes, Avatar Korra's uncle, and the Dark Avatar. Despite how terrible he was as a villain, he's still arguably the most powerful waterbender in the verse.
Speed and Mobility
When it comes to combat, speed is one of the most important attributes to have, and while he may not look it, Unalaq is one of the fastest people in the Avatarverse.
When it comes to travel speed, Unalaq is one of the best benders in the verse, just below the likes of Void Zaheer and Wan, and in terms of combat speed he's dead even with Korra, who's got some of the best speed feats in the verse. What's more, unlike most benders, he can combine his travel speed with his offense and defense, creating a highly mobile powerhouse that took a fully realized avatar to match.
In addition to his excellent speed, Unalaq is also a very powerful bender, both in terms of scale and potency.
While Unalaq doesn't quite have the absurd scale benders like Korra or Roku do, he's still one of the best in the verse when it comes to using his power in combat and combining it with his speed.
In addition to being a very fast and powerful bender, Unalaq is also one of the most skilled waterbenders in the verse.
Unalaq's shown a high degree of mastery over a wide range of different techniques, as well as creating a new sub-element, which is more than enough to say he's right near the top of the food chain as far as skill is concerned.
As his almost unrivaled combo of skill, speed, and power would suggest, Unalaq has a great track record against named opponents.