I love historical fiction that feels like the author knows his or her stuff, but doesn't show it off too much. Some of my favorites:
Dream of Scipio and An Instance of the Fingerpost - both by Iain Pears. Some people find Fingerpost slow going, but I really loved drowning myself in it. It's set in the 17th Century and if you're willing to go with it for a while, you'll come away happy once you realize where it's going. Dream is set in three time periods: the 5th Century, the 14th Century and WW II. It's incredibly engaging, but you don't have a clue as to how the story is going to come together until the last chapter and then, suddenly, it all snaps into focus. It's one of my favorite books of all time - incredibly sad, passionate and very, very smart.
The Baroque Cycle - I've met plenty of people who can't stand Neal Stephenson's massive (2000 + pages), three volume historical novel and there's nothing wrong with that. As for me, I couldn't put it down. Full of swashbuckling, algebra, lost innocence, Thief Kings, plots to topple the crown and the invention of money it's one of the biggest and most satisfying historical novels that really earns the title "epic" that I've ever read.
Tipping the Velvet and Fingersmith - both by Sarah Waters. When she's on her game, no one does historical potboilers better than Sarah Waters. Fast-moving, page-turning and dishing out some of the most authentic female characters in historical fiction, these are like candy bars for historical fiction lovers, only they're low in calories and high in nutritional value.