I always like the idea of giving the first book in a series to get the child "hooked". My "average" (also perfect of course, but I'm not biased) son, at 8 liked the following series:
Little Wolf by Ian Whybrow
Dragon Slayer's Academy by Kate McMullan
The Heroic Misadventures of Hiccup the Viking by Cressida Crowell
All of which are on the advanced end of the "easy reader" level (e.g. a smidge more challenging than Magic Treehouse, but the same slender size) All of which are "fantasy" but with a goofy little boy slant and not at all didactic. I actually read all of them to him and found them pretty funny myself. However I think many 8 year olds would be able to read them themselves.
Three other series that we really liked at 8, but definitely were read alouds for us, were Bunnicula, the Magic Shop Books, and Dealing With Dragons. You'll notice a pretty strong fantasy theme here because that's what my DS likes, as do most of his friends and classmates.
If you're looking for something showier, the Dragonology and Wizardology books which DS could spend hours pouring over at that age.