I can't believe the logic being thrown around here. Unfortunately for a few of you, "best" is a subjective superlative, so all we're arguing here is opinion.
The following are objective statements and immune to argument.
FASTEST marathon? London (or Chicago)
OLDEST marathon? Boston
Now if we're debating the "best" marathon, I find it hilarious that some posters point to the big names and fast times at London over the last 4-5 years [how far back does London go anyway - were there any meaningful "Londons" 10-20 years ago?]. As many of you have noted, people like Geb and Tergat will go to London over Boston because of 1) the purse and 2) a greater chance of setting a WR on the flat course. Unfortunately, the current elites pass on truly testing themselves at Boston, as they would have to run - as Pre would put it - a "pure guts race". Ask Salazar how he felt about his running career after his "duel in the sun" with Beardsley.
Also if the Dubuque (or East Bum, or wherever) 10K Turkey Trot suddenly announced a $1 million award for first place and all the A-list Africans flew over to compete, does that make the Dubuque 10K Turkey Trot the "best" 10K in the world? Of course not. History has to be considered.
History is what makes Hayward Field special. And Madison Square Garden. So to criticize Boston for "resting" on its history is ludicrous. Boston kept alive the marathon-distance competition for decades, keeping it in the public consciousness.
Ugh, I could go on for pages, but it's silly I have to argue this. How about this: the Boston Marathon is to the World Series as the London Marathon is to the World Baseball Classic. Both have big names; one has history.