The flattest marathons in the US are Houston and Chicago.
The flattest large marathons, you mean -- I've run at least two courses flatter than Houston off the top of my head (Skagit Flats and Foot Traffic Flat); there are probably others. There are actually quite a lot of overpasses and underpasses on the Houston course. Add in the all cement for some pounding, and it doesn't really feel that fast to me.
[Competitive] Jerk
Sorry, I meant large marathons, correct. There is actually a small local here in Fort Worth, the Fort Worth Marathon that is as flat as marathons can get, out and back next to a river on a flat trail with one low water crossing as the only climb. You can run it either on concrete or dirt. Too bad that weather has been an issue for that race for the last 3 years (70F+ and high winds) plus you better carry your own water as the organization is horrendous.
When I think marathons I thing large marathons that are worthy of a trip or smaller local ones. I can't really imagine flying out somewhere just to run a small marathon. But that's just me.