So hard to pick, but I have been lucky to try out some pretty great ones and have been impressed with a lot of them. Some of my favorites...and I can't wait to do the MCM in a couple weeks because I have a feeling it will be added to my list of favorites.
Kiawah Island Half Marathon, SC: Beautiful course, all water stations have oranges and bananas, after race food is fantastic as is the organization and medals are recycled glass. Running the Marathon this year since it is a new course.
Air Force Marathon, Dayton, OH: Amazing support, running on an AFB is pretty cool, having a flyover (B2 Bomber this year) and various flyovers during the race, mile 10 is AMAZING!!, and running by the jets at the Air Force Museum is nice for your approach into the finish line!
Lincoln Presidential Half Marathon, Springfield, IL: What other race can you get a picture with President Lincoln and his wife with a giant penny medal around your neck?

Plus, you get to run through all the historic areas, Lincoln's house, tomb, and the capitol. It is a very nice city and a great race.
University of Missouri (Mizzou) Go Girl Half Marathon, Columbia, MO: I admit I had never even heard of Mizzou before or Truman the Tiger and had never even been to that state, but I got my picture with Truman.

Although it was hot, the support was great, the goodies were nice, and the course wasn't difficult. A wine glass with a smoothie and a nice medal only added to how fun this was.
Chicago Women's Half Marathon: The course was run almost entirely by Lake Michigan and like most women's focused races, you got a lot of goodies with this one too. I really loved the course (although it was hot and Lake Michigan was ironically not windy) and the support and the atmosphere.
Rock 'n' Roll Chicago Half Marathon: I love architecture, and the course is run by some of the coolest buildings, and having bands every once in a while to get us through what ended up being a hot race was nice. A lot of crowd support and the Expo was amazing! Good thing we drove because we got so much free stuff. I liked the support on the course, and the overall organization.
Lady Speed Stick Half Marathon - Nashville, TN: Although the course started out really hilly, it eventually turned into a nice downhill and flat course. This series was acquired by Competitor, so it is now associated with the RNR events. The Expo wasn't much, but the course and medal were amazing.