For me personally I was always bothered by his spelling of 'loth'. Tolkien spelled it 'loth' but Christopher edited it to 'loath'. And everytime I reread 'the Silmarillion' I can see how cut and paste it is, especially the chapter 'On the Sun and the Moon'. Despite these things bothering me I am grateful.
His greatest achievement in my opinion was 'the History of Middle-earth' series. In these 12 volumes he showed his skills as a Tolkienologist, giving us commentary and notes on Tolkien's previously unpublished drafts. This is where Christopher truly shone.
Thank you Christopher