Aside of the already mentioned there is always the obligatory Dresden Files recommendation (which, of course, got recommended while I was writing this post... go figure). The first few books are more of a noir urban fantasy mystery story, but around book four the fantasy side slowly gets more and more dominant until the magical detective thing is completely forgotten. Also one of the few series that somehow manages to get better with each book.
Also from Jim Butcher, there is the obligatory second reference of Codex Alera, a heroic fantasy series concerning Romans with elemental powers fighting against (or with) Neanderthal elves, psychic yetis, blood-mage werewolves and the zerg. Very fun with lots of high-concept ideas and crowning moments of awesome, though it can sometimes get a little bogged down on the secondary characters and their sub-plots.
The Belgariad was already recommended (by KyuubiNoKitsune-Hime, The Pawn of Prophecy is the first book in the series), and I would also give it a lukewarm recommendation. It is a pretty straightforward heroic fantasy story that intentionally revels in all the clichés of the genre (the protagonist is heir to dead line of kings who grew up as a farm-boy, for starters...), and while it is an entertaining read all right, it has quite a number of cringe-worthy plot-devices (like the fact that there is an in-universe "destiny" that is pretty much the author's avatar moving people, often against their will, to cover any plot-holes).
I also recently read the first book in the Old Kingdom series, and it was actually really good, with an unusual setting (it has a magical land in a medieval stasis and with a serious undead problem bordering a 1920s technology level country that really, really don't want to do anything with them) and an unusual protagonist (Sabrielle is a necromacer who uses magical bells to bind and banish undead and she has to go into the Old Kingdom to find her father who might or might not be dead).
For sci-fi, I would say anything from David Weber, and I specifically recommend his Dahak trilogy (or rather duology, as the third book is not really part of an arc), as it is shorter than his other works and shows off a lot of his affections for character-building and very large-scale space battles. The premise is that humanity on Earth is the descendants of a spaceship of a galaxy-spanning human empire that had to be abandoned because of a mutiny on board. Said spaceship is the Moon. As in, literally the moon, which swings into action when it detects that an alien fleet is nearing the border of the old empire and thus its artificial intelligence kidnaps a US astronaut and forces him to become its captain so that it could end the lockdown set by the previous captain and warn the empire... and then things get much, much more complicated.
My other two recommendations are linked: Gaunt's Ghosts and Ciaphas Cain. Both are set in the WH40k universe, both star great commissars respected by their men, but that's where all the similarities end. Gaunt's Ghost is more of a traditional story about war in the 41st millennium, while the Ciaphas Cain novels are more of a tongue-in-cheek representation of the grimdark universe told through the memoirs of the aforementioned self-deprecating commissar who always just wamted to get a cushy desk job as far from the front lines as possible, yet due to his own misfortune (or fortune, depending on how you look at it) he not only keeps getting into the thick of it, but also manages to cultivate a heroic persona while all he is trying to do is staying alive (or at least that's what he says, even when he is doing incredibly selfless stuff left and right... the novels have a bit of an unreliable narrathor thing to them).