I've come to the conclusion that there is no best. Each ruler has something different to offer and the value of their qualities are entirely subjective. In the end it boils down to this.
Cerys
Democracy: Check
Unity: Check
Military Strength: Fail
Hjalmar
Democracy: Check
Unity: Fail
Military Strength: Check
Svanrige
Democracy: Fail
Unity: Check
Military Strength: Check
I see it more like:
CERYS
Unity: 8/10 (Madman Lugos is out)
Prosperity: 10/10
Military: 5/10
HJALMAR
Unity: 8/10 (Madman Lugos is out)
Prosperity: 5/10
Military: 10/10 (Constant raids)
SVANRIGE
Unity: Excellent (All Clans approve even an Craite)
Prosperity: 8/10
Military: 8/10