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 Fantasy. $6
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.