I doubt if single sign on is more secure than having a separate signon. After all, if the password in your PC is hacked, that gives access to the cloud also if you use single signon. While if you have a separate signon, the data in the cloud is inaccessible for a hacker.
Single sign-on is for convenience, not for security.
And for the best, first formulate your criteria, then investigate which of the three is best. If no criteria, all seem to be best.
How can we secure our Organization cloud data? and choose the best Single Sign-On option among the CloudCodes, Okta, OneLogin?