Ok, so everyone seems to agree on The Wheel of Time and Mistborn. Sadly, I can't agree with either because I've only read the first two books of the former and haven't picked up the latter yet. As for BEST books, i.e. the ones I keep coming back to... I noticed one lone reference to The Deed of Paksennarion by Elizabeth Moon which I have to second enthusiastically! Recently I've also really enjoyed her Kylara Vatta series (sci fi).
I also have to mention Madeliene L'Engle for her series beginning with A Wrinkle in Time, and Georgette Heyer who writes Historical Romance novels devoid of text-based porn. Her characters really shine and while the stories are somewhat predictable in that they follow the accepted plot line- guy meets girl, they appear to detest one another yet end up falling in love- not all of them do, and they are a very enjoyable read with a lot of laughing involved. I would recommend starting with These Old Shades. It's as much mystery and adventure as it is romance, with a delightful anti-hero.
As for more modern fiction: Jasper Fforde with his Thursday Next series. Anyone who loves to read will like these books, I promise! The Literary Detectives FTW!