I'm just going to categorize whole series into one book, if you don't mind :) In no particular order:
Lord of the Rings by J.R.R. Tolkien: You can't find writers like that anymore. It's just that mystical, amazingly fantasy quality in Tolkien's writing that draws you in deeper and deeper into the book till you get lost in his perfect world of elves, humans, orcs, dwarves, hobbits and more.
Bartimaeus Trilogy by Jonathan Stroud: For a contemporary writer, he incorporates both a great sense of humour, sarcasm and wit into his trilogy. Think djinni, imps, ifrits galore and more, coupled with a somewhat modernized century but still trapped in the past, plenty of laughs and surprisingly, you get a great read for teenagers and adults alike.
Jonathan Strange and Mr. Norrell by Susanna Clarke: Two words to begin with - it's long. Make it three - it's VERY long. But one of the best books I've ever read. It starts off slowly, and I believe that many people don't make it past the 150th page mark. But I perservered and boy, was I rewarded :) Another fantasy genre book, but based on England and the revival of magic and wizardry. I got lost in the book for almost the entire of summer, and only came back to the world when I flipped the last page :(
The Good Earth by Pearl S. Buck: It's strange, how uncannily accurate Buck manages to describe rural farm life and rituals in 18th to 19th century China. Of course, given that she lived there, but the way she has written the book transports you back to that time and era, so much so that you can imagine every little detail in your mind, and envision the whole scene playing in front of your mind.
The Kitchen God's Wife by Amy Tan: Perhaps it's my Chinese roots that draws my to Amy Tan's books, but this one draws me in the strongest. It entails how a mother reveals her life long story to her daughter, unravelling secrets that she had kept hidden from her adult daughter for 30 years or so. It is very touching towards the end, and made me reflect alot.
There you go :) Maybe you'd give some thought to reading some of these books. I actually have some more books that I would like to categorize in the top 5 genre (mostly classics and plays) but I thought giving light to some of these lesser known books might be more fun.