Well, Shaak Ti took out countless Magnaguard's in the Clone Wars Cartoon while Saesee Tiin got pwnd by Sidious in one second. But he did take over a separatist Starship in the Clone Wars Cartoon with the help of some Clone Troopers, still, not as good as taking out like 10 Magnaguards while fending off 10 more. And Shaak Ti utilizes Makashi, so she has an edge over Saesee (anyone know what form he uses?)
Well let's think logically. Most Niman users died at the battle of Geonosis. Kit Fisto is the only known Shi-Cho master at the time. Makashi is an ancient saber form and only a few jedi ever had a need to master it. Juyo was considered an incomplete form and I don't know any jedi who mstered the incomplete form. Even the saber instructors mastered every form with the exception of Juyo. And he clearly didn't master Vaapad. So this leads Ataru, Schien or Soresu (in my eyes), and judging from how he fights in the clone was cartoons, I'm pretty sure it's Schien or Djem So.
Anyway, when it comes to forms the only ones that makashi has an advantage over are Niman, Shii-Cho or Soresu, and he could have possibly mastered Soresu though I doubt it from the way he fights in the cartoons.
Wtf! Makashi is better at duelling than any other form - except possibly Vaapad. Duelling is what it was built and bred for. Djem So is beleived to be good against Makashi because of its brute strength. However I dont think this is proven...
Makashi is a great form, don't get me wrong. It was bred for graceful dueling and has elegence and class, however when up against an overwhelming form such as Djem So/Schien, Ataru or Juyo/Vaapad, it's really not too impressive as the user is not able to maintain the concentration that is required to utilise Makashi effectively. Remember, not every Makashi dueler is as great a dueler as Count Dooku. Count Dooku is extremely experienced and would thus be able to contend with almost anybody, superior form or not but someone like Shaak Ti would easily get overwhelmed by a Schien user imo.