I'm going to judge based on users.
Almost no-one at the top tiers uses Shii-Cho, and Yoda's the only high-level who seems to make Ataru work as a primary form, so they're out.
Makashi has Dooku and Shaak Ti, which is pretty good, but some of the others have stronger lineups.
Soresu, Obi-Wan, Zannah. Also not bad.
Juyo/Vaapid, of course has Mace, but behind him, there's Maul and not many other notables who use it as a primary form, though most of the best have at least some training in it. Galen Marek had it as one of his two mains too.
No weaknesses seems to work out pretty well once one gets to higher levels. It might not be the best for the mid-rank, but quite a few high level users do use Niman. Exar Kun, Darth Krayt, and... well, Exar Kun and Darth Krayt.
Djem So has Bane and Anakin and Kyle Katarn, though, plus Luke using it as one of his primaries, topping the rest, IMO.
(And not really including practitioners like Palps and Kas'im who didn't favor any form)
So gonna go form V here. While any form can be dangerous in the right hands, it's a favored choice of more high-end users than the others.