1. To all of you Niman haters out there...you are going to love this.
"However, it is my unofficial opinion that Revan would generally have used "Form VI - Niman". This form works well for anyone who is intelligent and adaptable, as Revan obviously was. It has no real weaknesses, and even though it is not as aggressive as other forms it enables the user to unleash powerful Force abilities more easily during combat. Revan was skilled in lightsaber combat, but knew true strength came from using all the other Force abilities in conjunction with lightsaber combat. Form VI would allow Revan to spend less time focusing on lightsaber skills, and more time developing other Force powers."
―Drew Karpyshyn (Star Wars: The Old Republic: Revan Author)
This was the guy that was the Senior Writer for Star Wars: Knights of the Old Republic at BioWare. So, technically this guy gave us Revan. This is supported by the next statement.
"...though he was skilled in all of them [lightsaber forms] - he was always more of a generalist than a specialist."
―Drew Karpyshyn (Star Wars: The Old Republic: Revan Author)
So basically, he was skilled in all of them. Being a Jack of all Trades, Revan became a practitioner of Niman. While displaying Ataru and Soresu against the Sith Emperor and Juyo as a Dark Lord of the Sith, I believe that other than Niman, Revan frequently used Shien/Djem So and also Jar'Kai.
2. As to what form I would prefer as a Jedi/Sith, I would have to go with Form V: Shien/Djem So. It's the most practical in my opinion and also the form that, as a Jedi, I could pummel the crap out of people and not get *****ed at for leaning towards the Dark Side. As a Sith, it has more defense than Juyo, which means less openings. And it does kick ***.