Because no one in High 6-A is High 6-A anymore, I'm nominating Count Dooku for that spot. He had a nigh-perfect fighting record and has been stated to be Windu's equal and sometimes even superior in terms of pure skill (and to be frank, has vastly superior skill feats in the other forms of lightsaber combat via scaling to his students, such as Grievous, Ventress and others), and though he lost in the end and was amped by a dark side nexus (that also weakened Yoda, meaning power was roughly equal then), he is capable of competing with Yoda in terms of pure skill, as shown in Dark Rendesvous. While Anakin was roughly equal to him at the end in terms of raw skill and superior in terms of power, I would still say Dooku would take this in terms of skill because he's an excellent tactical and strategic fighter, as well as being very creative. Vader is about equal to Dooku in terms of tactics and strategy (I'd even argue superior), but has less impressive skill feats, so I suppose they could share the spot?
On the topic of Yoda, he should honestly be there in 4-B instead of Sidious or Luke. Luke's good, but he never surpassed either of them in terms of pure saber skill if you ask me, given his skill feats are considerably inferior to either, and he only defeated Sidious via an amp from Leia. Sidious is acknowledged to be somewhat above Yoda in terms of force power, but Yoda actually disarmed Sidious of his lightsaber, proving he is the better duelist in terms of fighting skill, and they both got the upper hand at times with clever uses of force power roughly the same number of times, so IMO Yoda > Sidious in just pure skill