If I could do any, I would look to do Boston (this one has qualifying times) as I love the city and the race has a good history behind it.
Or, if you want a memorable and tough course, then the Athens Classic Marathon would be on my list. Starting in the town of Marathon, and finishing in Panathinaiko Stadium in Athens. The real thing!
For me, I did a low key one first, less than 1,000 competitors, loads of support, fantastic rural scenery, not crammed like sardines in starting pens etc, fantastic organisation, easy transport / parking etc etc.