It most definitely I'd the best defensive justu. It's hard as hell to break through only one person has ever hit someone through susanoo and that was the raikage and hes a beast when it comes to strength he has the chakra of a tailed beast. But he only got through an incomplete susanoo both times (sasuke rib cage and madaras unarmored susanoo) and the second time if oonoki didnt help him he would not have pierced it.
It took the combined effort of two kages to break madaras susanoo that's no small feat. It protects the users body all around and unless he is standing in sand gaara can't pull the user out. It's so strong that it was able to protect Itachi from one of sasukes most devistating justu (Kirin) and it didn't even form completely. Not only is it an awesome defense but can be used for offense, an offense that took the combined efforts of of oonoki and gaara to stop.
As for it only being able to guard from the from I don't think that's true. Itachi used it to guard against Kirin and that came from the sky and I'm pretty sure did damage all round him since it demolished the uchiha hide out. It protects the user 360 degrees when fully formed.
Thanks to the recent chapter we know it does not limit the users mobility at all. Madara was able to move fast enough to surprise the kage and bitch smack them all to the ground. Madara was able to take an sage mode odama resengann and not even be affected.
It activates instantly as seen when Itachi activated it for the first time, sasuke stop every bone in his body from being broken by the raikages liger bomb at the last second, and Madara activated it a split second before being hit with the sage mode odama resengann.
And not only does it protect the user but it can assist in hand seals making it easier to do certain Justus. With the blaze release it can be covered in amerturetsu making it even more dangerous to deal with.
So to summarize susanoo near impenetrable, fast, can be used for both defense and offense, can be put up in an instant, and is armed with weapons of its own (bow and arrows, double swords,etc) plus what ever other abilities the EMS givenit that we havent seen, no other defensive justu can even compare. While it does have its weaknesses which are very few it is still bar non the best defensive justu. Just because one of the strongest ninjas in naruto manage to get through it doesn't make it weak at all.
And regarding madaras susanoo it is incomplete I'm pretty sure when he puts the armor on it it will be on a different level. Some say it is complete because it's double sided but if we look at both Itachi and sasuke we can deduct that it is incomplete.
Oh ps I forgot about oonokis "IMA FIRIN MA LAZAAR" Justus but even still that is an s ranked justu.