Well, the whole Loyalists within the Traitors have been kinda covered. Loken, Garro, Saul Tarvitz. Though those was kinda before they went into the deep end. I remember the scene where Iacton is fleeing from the Vengeful Spirit with Euphrati Keeler just as Horus executes the Remembrancers, where his loyalty to Horus finally snaps.
Though it should be pointed out that many of the traitor legions had gone through purges to weed out loyalist elements, and Horus in particular had sent the Sons of Horus against the Interex after he sabotaged the peace efforts, to blunt the impact of fighting astartes in the future, due to them using a version of powered armor. But once the renegade legions began to openly walk the path of damnation, its hard, if not downright impossible to remain untainted and getting anything more than a bolter shell in the brain from the Loyalists.
And for the other legions..
Word Bearers: Nearly utterly corrupt, with possible fringe elements untainted because they had been left to uphold the charade.
Thousand Sons: Driven into the Eye, but might have untainted fleet elements here and there.
World Eaters: Nailed hard and led by a demon prince.
Death Guard: Already have had loyalist elements fleeing, a very corrupt fate comming up for the rest soon thats going to leave it pretty much impossible for them to retain any loyalists with Typhon mirring them in the warp.
Emperor's Children: Of the deep end coo-coo. Just look at what they have become in Angel Exterminatus.
Alpha Legion: One of the few maybe legions. But with their serpentine planning, any 'supposed' loyalists from them might just be faking it.
Iron Warriors: Bitter and spiteful, but not yet truly having embraced chaos. A definitely possibility here. Just look at Warsmith Dantioch getting in with the Ultramarines. We might see him again in Unremembered Empire.
Night Lords: If going by Prince of Crows, they are not in the deep end yet. And Ive a gut feeling that the true reason for Sevatar's disappearance is that Malcador might give him a rather interesting offer, while he is in the custody of the Dark Angels. He is after all a rather strong psyker.