steam: I think the Castle looks classiest. I'm a bit of a GWR fan as well I'm afraid, and rarely salivate over non-GWR locos, although I do like the majesty of the Britannia.
diesel: I've always had a soft spot for the Class 47, but never had one. my layout is based on mid-late 50s, but I might be able to squeeze a cheeky flash-forward to early 60s BR Green.