In hindsight, which Primarch would have been best for Eldrad to approach?

Eldrad approached Fulgrim to try and warn him of the heresy. Unfortunately, She Who Thirsts already had her claws too deeply in Fulgrim and it did not end up well.

Which Primarch would have been the best for Eldrad to approach?

Can break it down into two parts. Who would have been the easiest to convince, as well as who would have been the most effective with the knowledge. (For instance, if he convinced Konrad, I don't think anyone would have listened to him).