Top picks:
Paris
It's a massive event with the potential for fast times, although apparently the elite race isn't super well organized. But they have the cash and the crowds. And it's in Paris.
Fukuoka
The informal world championships for many years. A weird, serious, small men's marathon (which is bizarre, and they may need to change that). But it's got some serious history.
Rotterdam
With more money and prestige, this course could produce WRs.
Problematic runners up:
Dubai
Fast, but also fast because of what I've heard are "questionable practices" (i.e., if you're young and doped and want to run a 2:05, this is your race). But it would be worth having a Major in that region.
Amsterdam
Big enough race, neat finish, flat, fast course. But it doesn't spend on the star power and seems like a bit of an afterthought in that region.
Toronto
Well organized, lots of ambition, but the marathon itself is very small and can't bring itself to spend on total rock star elites. Like Amsterdam, Frankfurt, LA, etc., it is more of a proving ground for those who function as training mules in big East African training groups than it is an attraction for the big names from those same groups.