Fantasy - China Miéville has some of this past decade's greatest fantasy under his belt, in my opinion, with his Bas Lag books (they're not quite a trilogy, as each is a self-contained story, but they are all chronological). Start with The Scar and see how you go - it's the most the reader-friendly. Perdido Street Station is a wilder, more sprawling book, and Iron Council is highly experimental. Iron Council is my personal favourite, but a lot of people don't like it, partly due to its structure and partly due to the politics (Miéville is an outspoken democratic socialist).
As well as the Bas Lag books, he has some great stand-alone fantasy as well, like The City & The City, Kraken, and of course his two excellent short story collections, Looking For Jake & Other Stories, and Three Moments of An Explosion: Stories.
Sci-fi - I might cheat and say Miéville again - Embassytown is a fantastic book about, in the author's words, a "linguistic apocalypse" on an alien border world. Iain M Banks' Culture novels are goddamn masterpieces as well, if they haven't already been mentioned. Player of Games is the agreed-upon best starting point, though I started with Consider Phlebas and enjoyed it just fine.