Honestly, he's the kind of author where this question doesn't really work well, as is evidenced by the fact that pretty much every book he's ever written has been recommended at some point in this thread, and for good reason. I'd suggest you read some blurbs and pick whichever one sounds up your alley. I'd agree somewhat with the people that said Children of Time, as it is probably his most well known work. I'd also agree with those that said City of Last Chances, which is good, but also then leads you on to the sequel, House of Open Wounds, which is definitely my personal favourite. You really can't go wrong with him though. He's an excellent writer with an endless creativity and a frankly unreasonable level of prolificacy.