Very difficult because there really is nothing quite like him. Presumably you have read Neil Gaiman - Anansi Boys is fantastic, also the one with TP - Good Omens, which is first rate. Have you tried Robertson Davies? The Deptford Trilogy is very good and although not funny all through there are some very well written comic scenes.
Nobody's as all-round wonderful as Terry Pratchett! But some of the later Tom Holt books (The Portable Door, In Your Dreams, Earth Air Fire and Custard, You Don't Have to be Mad to Work Here But It Helps, Barking) are well worth a look. They're funny, and unlike some of his earlier books, have involving characters and a coherence of plot.