1. Father Brown Omnibus
Crime/Mystery. Free. Public domain.
2. David Balfour (Kidnapped & Catriona)
Adventure. Free. Public domain.
3. The Misenchanted Sword
Fantasy. $6 or less from Fictionwise
4. The Deed of Paksenarrion
4. Cordelia's Honor
Science fiction. $6
Just five? Here's a randomish five from ebooks I've read. It's impossible to rank all the ebooks and pick the absolute top five - it would take hours, if not days.