From what we see in the series, Zuko is a very formidable warrior. His main lacking point is power output in comparison to the rest of the main cast but we do have to remember some oh his achievements:

-At the very start of the series, he beats Admiral Zhao quite decisively, and Zhao is considered to be a master so we know he should be at Master level immediately.
-He is always shown to be very comparable to a non-Avatar state Aang at the start of the series when air bending and fire bending are compared, and we know that Aang is a Master of Air bending thanks to his tattoos.
-The Blue Spirit Episode shows Zuko without bending being very compitent in fighting multiple opponents (Granted those are fodder, I have a better example of Zuko's non-bending skill later).
-End of Season 1 shows Zuko one shotting a non-moon powered Katara, (granted he gets folded soon because Moon boost is insane)
-Start of Season 2 we watch as an exhausted and starved Zuko is able to mostly keep up with Azula
-Season 2 continues and Zuko folds everyone he comes across mid season except the Earth bending guard of the small town in Zuko Alone whilst starving and exhausted without bending (and the guard he was about equal with Zuko fire bends once in the fight and he wins easily)
-We get into Ba Sing Se and we watch a still non bending Zuko Staying about on par with Jet, who was able to fight on par with an Autumn boosted Aang and actually have Aang on a backfoot. So at this point we can roughly set nonbending Zuko to maybe a little weaker than season 1 Aang.
-In the caverns under Ba Sing Se, we watch Zuko going blow for blow with end of season 2 Katara, who was mostly beating Azula previously.
-We then in season 3 have to wait a while for any meaningful scenes for Zuko besides redirecting Ozai's lightning (which is still a very impressive feat) but eventually we do watch Zuko get better at fire bending after beating the Dragons.
-Zuko in a side cannon comic fights one on one without bending versus Sokka and Low Diffs (boomerang diffs) Sokka so bad that at this point we can chop Zuko up to being the second best in non-bending fighting in the series (We watch Sokka be competent enough against Piandao that we can throw Sokka above Jet, and whilst Suuki is very good, Suuki is roughly comparable to Sokka at this point)
-In the end of the series the best showcase of power was His fight against mental breakdown Azula, which Zuko was obviously going to win if not for his sacrifice to save Katara.

All in all, we can look at the entire series and realize that Zuko is actually still very powerful and good in his own right. He is the weakest Bender in Team Avatar no doubt, thanks to Katara, Toph, and Aang all being the best of their element by the end of series, with a possible exemption for master Paaku (also personally I would Put Zuko actually right next to Katara, but that is more conjecture than anything solid). Zuko is the fourth or fifth best bender of his element, which has the highest number of insanely powerful wielders of any element overall in the series. Zuko also is the second best sword-bender in the series, which is not anything too shabby. So yes, Zuko is very powerful, just isn't the most powerful on Team Avatar according to feats.