Let me explain what I mean by this. We simply replace Artoria with one of them. We keep the main events of the Fourth Holy Grail War the same, but change the outcome. That is, whether any of them would come into conflict with Kiritsugu or not. And who would be able to handle the six enemy Masters.

Nobody.

Gilgamesh is still there, and all of them are much weaker than him (and unlike what happened with Saber (Artoria), he is not going to hold himself back against any of them). Moreover, Siegfried has a major weak point that Gilgamesh can use against him (and Gilgamesh is aware of that weak point). As for the others, they're still much weaker than Gilgamesh.

By the way, who is stronger — Arthur Pendragon (as Saber), or Artoria (as Saber)?

If you mean the relationship with Kiritsugu, I think it would be like this, more or less:

Arthur would act more or less the same way as Saber (Artoria) does.

They're not that different in their personality (at least as I understand it).

Siegfried would follow orders even if he doesn't agree with them.

And I don't know enough about the others.