This is a very difficult question to answer.
Tatooine Luke is the essence of the character. Optimistic, idealistic, kind, compassionate and humble. It’s from Star Wars (or A New Hope, if your prefer), one of the most successful movies of all time, so it’s hard to go more iconic than that. There an unmatched simplicity to this version.
I’m not particularly fond of the Bespin suit, but it’s the one he wears in the duel with Darth Vader, still the greatest lightsaber duel in any Star Wars movie. It may not look the best to me, but it has power due to the story.
Jedi Knight Luke is an obvious choice. At the top of his powers and what he used while he saved the Galaxy. It’s classy and elegant.
Young Jedi Master (or “Crait Projectiion” if you will) is a variation of that. Both dark and white while not being as elaborate as the Republic Jedi uniforms. This represents his greatest failure.
The hermit costume is what he was during his last years. Old and full of regrets, yet he developed a charming sense of humor.
And lastly, the Old Jedi Master. The one he used when he dies, once more giving hope to the Galaxy. A white and pearl Jedi robe, going back full circle to his Tatooine roots.
They are all great. They are all aspects of what is, in my mind, the best Star Wars character. I can’t pick one so I’ll pick three: the beginning (Tatooine), the top (Jedi Knight) and the end (Old Jedi Master)