Ok, i read all, but sorry i've to disagree with you. Not that you are totally wrong, but i would like to say that there is no really best defense. Every defense has his advantages and disadvantages.
First of all you must know there are different types of Susanoo. So it depends now about which Susanoo you are talking about. Itachis Susanoo with yata mirror for example makes it hard for almost any attack to go through it. Madaras Perfect Susanoo is just too big and too strong for that a simple ninja with katons, suiton, fuuton or even with raw strength for example could break through it. When we've already seen that part1 Naruto(with Kyubis chakra) for example was able to do damage on Neji eventhough he released his kaiten just with one punch. And that's when we forget the fact that in the contrary of Susanoo or Gaaras sand, Neji must know when to use his rotation and he has to do it. So if he is surprised by an attack, he can't release his kaiten fast enough to protect himself.
Anyway, Gaaras sand was also penetrated by multiple occasions either by speed or by penetrating jutsus. Things that i hardly see happening against Madaras Perfect Susanoo(it protect from every side, so speed is useless) unless the attack is also significantly destructiv. For these reasons then, if we only count the defense aspect, Madaras Susanoo is better. So if i were to put up my list about which defense protect better, i would say it goes like this:
1- Madaras Susanoo
2- Itachis Susanoo
3- Sasukes Susanoo
4- Gaaras sand
5- Nejis kaiten
Now the problem with Susanoo is that it takes a ridiculous amount of chakra and i believe that Perfect Susanoo takes even much more chakra than a normal Susanoo and it's probably the reason why Itachi could not awake it(he doesn't have a great chakra). But that's if we talk about Perfect Susanoo. A normal Susanoo is enough in a fight. I'm not sure about this one, but i think Neji said in part1 that Kaiten also takes an important quantity of chakra to be released, not like Susanoo obviously, but it does take a great quantity also. Gaaras sand in the contrary takes no chakra at all since it's automatic. So in terms of chakra used on each defense, Gaaras sand is better
1- Gaaras sand
2- Nejis kaiten
3- Susanoo(depending on the version the user decides to use)
PS: I think you forgot about Tobis defense when i believe he is the one who has the best defense in this manga