Found this interesting enough list on Abe books and as with any such list its great fun to poke holes in it . Personally don't see how Good Omens isn't there but hey -
Top 10 Funniest Books According to AbeBooks.co.uk Customers
1. Right Ho, Jeeves by PG Wodehouse (1933)
2. Catch-22 by Joseph Heller (1961)
3. The Hitchhiker's Guide to the Galaxy by Douglas Adams (1979)
4. Three Men in a Boat by Jerome K Jerome (1889)
5. Wilt by Tom Sharpe (1976)
6. A Confederacy of Dunces by John Kennedy Toole (1980)
7. Lucky Jim by Kingsley Amis
8. The Code of the Woosters by PG Wodehouse (1938)
9. Bridget Jones's Diary by Helen Fielding (1996)
10. Adolf Hitler: My Part in His Downfall by Spike Milligan (1971)
Top 10 Funniest Books According to AbeBooks.co.uk Customers
1. Right Ho, Jeeves by PG Wodehouse (1933)
2. Catch-22 by Joseph Heller (1961)
3. The Hitchhiker's Guide to the Galaxy by Douglas Adams (1979)
4. Three Men in a Boat by Jerome K Jerome (1889)
5. Wilt by Tom Sharpe (1976)
6. A Confederacy of Dunces by John Kennedy Toole (1980)
7. Lucky Jim by Kingsley Amis
8. The Code of the Woosters by PG Wodehouse (1938)
9. Bridget Jones's Diary by Helen Fielding (1996)
10. Adolf Hitler: My Part in His Downfall by Spike Milligan (1971)