I'd say Abingdon - never done it myself but know plenty who have who got a PB. Would agree inaugeral races are not always a good idea, and ones along sea fronts are very weather dependent.
I did the New Forest Marathon twice in my early years of long-distance running, is a beautiful course and great if you don't mind running stretches on your own ... BUT it does have some hills, and they are mainly in the latter section; plus that weekend is traditionally quite hot weather-wise, and there are long stretches on open road. Personally I like a few hills to break it up for the the legs - use different muscles etc, but that's just me.
We have a VERY flat marathon up here (but it involves a flight from the South of England!), the Moray Marathon has provided many a PB for those local enough to complete it ... and it's in September.
Edited: 19/12/2012 at 16:45