What’s the one must-run marathon that should be at the top of my bucket list?

Just depends what you want. Fast time - Berlin or Chicago or London. History - Boston or London.

If you want scenic or small town maybe a wmm isn't best, etc.

The 5k marathon.

Olympic Trials

Flying Pig Marathon. Cincinnati, OH. First Sunday in May. GOAT

That is grasping for low hanging fruit.

OLYMPIC GAMES

Pikes Peak Marathon

I've heard that Marine Corps is supposed to be a good one, but can't confirm.

I've done all 6 WMM. IMHO, running a PR in any of them is extremely difficult due to logistics involved (having to get to start area 2 hrs before, late start for most of them other than Chicago, fighting crowds unless elite). If I had to pick one for the experience it would be a toss between Boston and London. Incredible crowds in both. Marathon vibe the whole week before. More historical landmarks on the London course but overall may be a slight edge to Boston for me.

Cheers

As others mentioned, this really depends on what you are looking for in a marathon. For example, NY is my least favorite to do - tremendous hassles getting to the start, trying to get a good position at the start, trying to pass all the people who should not have put themselves that far up in the field, etc. (The last time I did it, I was on one of the buses that were in a 5 bus collision getting to the start - and this was after standing in a line for well over an hour by 42nd st to get on the bus - who needs all that before a marathon?! I vowed "never again!" for NY!) Yet I know people (especially those who are thrilled to have people cheer for them) who *love* NY -- I volunteered for it for many years, and many runners seem to be enjoying themselves. So think about whether you care about having people cheering for you, or having a hassle-free day, or being treated very well by race organizers, or running a fast time (these are not all mutually exclusive, of course). Personally, my favorite 2 marathons were Montreal -- the race organizers treated runners *great*, it made NY look laughably shoddy in comparison, and it was conducive to running pretty fast -- and Harrisburg, which was low-key, incredibly hassle-free and easy to get to the start, and just about running a good race.

you found montreal "conducive to running pretty fast"?? what year did you run? for at least 13 years there have been some major hills in the middle (toward the end in the half), not at all fast compared to the fast ones named above...

Houston is incredibly well organized and fast. I stayed a <5 minute walk from the start (and it wasn't super expensive to stay there). Weather was perfect and an American record was set the year I ran it. Pace groups were awesome. Nutrition on course was exactly as promised. Basically, there wasn't a single hitch or flaw in the entire operation. So if running fast / setting a PR is on your list of marathon goals, I'd put Houston near the top.

I should say there was a HALF marathon record broken the year I ran it. But the marathon course was very conducive to fast running as well.

Honolulu. Seriously.

Another fan of Harrisburg! I also enjoyed Buffalo. They're similar - well-run, organized, logistically simple. I picked up my bib the morning of the race at both - I appreciate that. Neither one is a big production with many spectators, but I don't necessarily need that.

Buriram

Columbia River Gorge marathon in Hood River Oregon. Absolutely gorgeous route, small, but not so small that nobody is running 2:25. Easy logistics. Do some water sports in Hood River after or stay on Mt Hood

Pyongyang

highly recommended

I never got to do Honolulu and regret it.

For the experience, Big Sur is special and very different than doing a major.

Marathon to Athens. Run from the Plain of Marathon into the modern city of Athens.