Gaiman's writing style is very versatile, so much so that I had to plough through a few of his novels and graphic novels (http://mis_nomer.blogspot.com/search/label/gaiman) to find his "voice". I think the short story "How to talk to girls at parties" in "Fragile Things" and "The Sandman" is distinctive of his voice on the novel-end and graphic-novel-end respectively.

Although "Stardust" and "Good Omens" are not typical of his voice, I enjoyed both very much.

I've read a bunch of Irving's novels (http://mis_nomer.blogspot.com/search/label/irving) and while I've enjoyed every one of them, I liked A Prayer for Owen Meany and The Cider House Rules the best.

I would suggest starting with The Cider House Rules — I started with that book and it kept me coming back for more.