Agree with other postings that it's a huge topic with lots of variables. Will just provide a couple extra bits I didn't see mentioned already.
1 - JFK 50 has a lot of history, for military, for POTUS JFK wanting the country to be fit, and it has a guy with 49 finishes who tried for his 50th finish this year but couldn't make the time cutoffs. Has some sections along the AT towards the beginning. And of course Walmsley has the CR. Think it's a mix of singletrack (AT), gravel tow path, and road, but it definitely trends easy/fast.
2 - in an appeal to simplicity, it can be desirable to have a race that simply runs a trail and is finished, instead of having silly out and backs and extra loops just to hit a certain typical distance, like 50k, 50 mi, 100k, 100 mi. Like hey, there's a trail, let's do the trail, and see who gets to the end first. One I just did like that is the Whiskey Basin 90k (ish) in Arizona. I've also read the pretty new "No Business 100" in Stearns, Kentucky is a single loop course. Also a certain appeal in the point to point runs. I think both tomorrow's Madeira Island Ultra Trail (MIUT) and the classic difficult Diagonale des Fous on Reunion Island are like that...could be wrong on the Diagonale.
3 - I've heard both Tarawera and Old Ghost ultras in New Zealand are crazy scenic, though as noted by previous postings depends on your preferred environs. In my home area of Northern Colorado, Nick Clark has been putting on a super scenic race with growing interest - the Never Summer 100k (closer to 64 miles I think) in the Rawah and Never Summer Mountains in between Fort Collins and Steamboat Springs.
4 - stage races like Marathon des Sables (MdS) should be discussed. Goretex Trans Rockies Run. I think there's a new MdS style stage race in Peru that was supposed to be pretty scenic, remote, and a valuable addition to a fairly scant South American trail ultra scene.
5 - I figured for a long time that Matt Carpenter's Leadville 100 record of 15:42 was about as untouchable as they came, like his Pikes Peak Marathon record...but looks like Rob Krar ran 15:51 last year.
6 - As far as GOAT, we'd be remiss not to mention Kilian. And the dude does some crazy skimo stuff (set a 24 hr vert record of like 77,000 feet) and speed ascents (his debated Everest summits, Kili, others). Never met the guy, but seen plenty of video and he seems humble and that he just really enjoys being out there. During periods, has raced really frequently at ultra distances and done really well. And gets lots of tough points for finishing Hardrock a few years ago running the last 85 or 90 miles or something with a dislocated shoulder or elbow.