I want to know what role is the highest role of SELinux.
If I am mistaken, can anyone tell me what roles are available by default in SELinux?
Welcome to ring 0! btw, you will need RBAC for role based restrictions. You did guess it? RBAC comes from Role Based Access Control, there is others too.
First of all you should really read about SELinux as it can be dangerous to just set/get privileges without knowing effects. See for example Hardened Gentoo Handbook.
I think that highest role is simply what you have given most access permissions...
There is other solutions too, for example grsecurity also has its RBAC system. Gentoo Handbook first states that
Here's article about RBAC in SELinux from IBM Linux Technology Center.
And here is few lines from Gentoo Handbook: