This is a tough one for me because of my feelings that PC was so completely underserved by virtually every episode and series he was in. For Series 8 I'd say that If you want "true WHO" I'd go with
Deep Breath because I believe it's Clara's introduction to "the new incarnation" of The Doctor.
Listen because (IMO) it's the best episode in the series.
Mummy On the Orient Express because it's quite unique although bittersweet and then either
Into the Dalek because it's in a more classic vein in keeping with the rest of the show OR
Robots of Sherwood because it's rather charming and a complete departure from what you would normally hope to see.
As for series 9..... I was so disappointed by the first 4-5 episodes I barely paid attention to any of them or the rest for that matter. It was in series 9 I came to the conclusion that they were going to misuse, mishandle and pretty much just throw away anything they could have accomplished with PC's talents and I just gave up in disgust. He had so much potential. He could have had the same kind of run that CE, DT & MS had and they bloody well just tossed it all into a garbage disposal, hit "crush" and took anything it spewed out and called it good..... What a waste, what a damn bloody shame..... what a tragedy of abandonment. Peter Capaldi deserved
so much more and although I've loved Moffat forever.... what he did to P. Capaldi made me embarrassed for both of them.....
But, of course, that's just me.