There are different ways to approach this question.
COMFORT: Bellagio
The book at Bellagio is all about elegance. Comfortable leather chairs, a perfect setting and positively lovely cocktail waitresses.
ATMOSPHERE: Hilton
The Superbook is absolutely gigantic and they have a contest every Saturday called Superbook Saturdays for $30 which has a decent pot and is a fun addition to the day. You will also have no issue getting a seat as there are over 200 available to race players.
RACE ORIENTED: Orleans
The problem with Orleans is that this weekend is the Horseplayer World Series so I don't know if they're opening up the race book to the public. Otherwise, the Orleans is extremely racing-oriented and the comp system is outstanding. One point for each $12 you spend is going to be about the best you'll find in Vegas.
Other good ones are Mandalay Bay, which is positively gigantic but the employees have been rude and they don't give a damn about racing when a Jamaican bob-sled race is about to go off. Good luck trying to get down on a race with three minutes to post if the 12:30 college basketball games are about to tip off.
MGM Grand is OK but they chopped the hell out of the book when they installed the poker room. Caesars' re-designed book is nice and it's in the middle of the action, not closed off from the rest of the casino. I've heard good things about Rio, Venetian is very nice and of course, Wynn is top notch.
Avoid NY/NY as its book was quite clearly an afterthought. I also have had generally positive experiences at Luxor despite getting into a heated argument with a waitress there during Game 5 of the NBA Finals last year when she wouldn't bring me a beer despite me having multiple tickets on the game and a desk full of losing tickets from a long afternoon playing the ponies. She said I needed some kind of coupon.
Here's a good site to visit with individual reviews of the books.
http://www2.vegas.com/gaming/sportsbooks/
Good luck!
NT