Everything XTC did up through... Nonsuch got re-issued.
At the least, I would recommend getting the Skylarking reissue, since the original US tracklisting deleted Mermaid Smiled in favor of Dear God, and while Dear God is a great song, Mermaid Smile worked far better in the context of the album.
Beyond that, the original US CDs just had bonus tracks added to the middle of the albums, where the side breaks were. The remasters move the bonus tracks to the end of the discs. Although I'm not sure how consistent this is. I don't think Nonsuch and English Settlement ever had bonus tracks, and I'm pretty sure the Big Express remaster leaves them in the middle.
Pitchfork says the sound quality is definitely better in the remasters, although nobody around here carries /new/ XTC stuff for some reason.
Apple Venus never got remaster because it's not particularly old.
Also, uh, does it make me racist if I'm confused by the local East Asian population's fondness for eating sandwiches that consist of a slice of wonderbread between two slices of wonderbead and nothing else, as well as thinking that McDonalds is really high quality?
Cathedral's not an emotion.