If you like Potter, you're probably ok with Fantasy. One of the best fantasy series ever written (in my opinion) is the
Belgariad series. All told, there are about
10 books, and all of them get 5 out of 5 stars. And if you're looking for a long, top notch fantasy series, you have to try Terry Pratchett. They man has written a book or two a year since the early eighties, most set in his "Discworld" universe. Hilarious, very well written, and there are more than 30 in the Discworld series alone. I would start with either "
Wyrd Sisters" or "
Guards! Guards!"
As far as SciFi goes, you might want to check out anything by Vernor Vinge, starting with "
A Deepness in the Sky", then "
A Fire Upon the Deep" and then "
The Peace War" and its sequels. I've also just finished Richard Morgan's two books - "
Altered Carbon" and "
Broken Angels" - some of the best Sci-Fi I've read in years. Then, of course, there's anything by Neil Stephenson, starting with
Snow Crash.
You might also want to check out Arthur C. Clarke's
Rama series - those are quite good as well, and there are 3 of them, if I remember correctly. And finally, there's Dan Simmon's "
Hyperion" series - there are a few of those as well, and they're really quite excellent and original.