Well World Guardian doesn't necessarily mean your job is to banish all gods -take Seren for example. I think Guthix saw into the heart of the PC and recognized that he/she would do whatever they felt was right. So if you feel that Saradomin is the best thing for the world, then see it done. If Zamorak, see it done.
Saradomin gets so much shut. The reason I still like him is because he is that he really truly believes that he is good and when it comes down to it, he wants peace -albeit tainted by his own personal aspirations. He will even admit he regrets some od his mistakes -there is no such thing as a perfect leader -even Guthix made some mistakes. And if it weren't for thw other gods would he still have made the same distasteful decisiin
Who else could truly bring us peace?
Armadyl? He is too weak, too naive. Seren? She isn't aggressive enough and cares little foe what's going on outside her bubble. Zaros? Peace at the expense of freedom. Additionally, he has made so many empty promises and failed to prive his goodwill through action. He only cares about becoming an Elder God. The godless? As already mentioned and even mirrored irl is that mankind tends to quarrel when left to thwr own devices. Sliske, Zamorak. Name is were lefr out for obvious reasons.
He's not great but he is the best decision for Gielibor.