All of my comments are through the lens of living in a spot where the ultra/MUT community is not particularly strong or widespread due to geographic constraints. But here is another attempt to push the conversation.
GOAT- In addition to the names already mentioned (Trason, Jornet, et al) you have to toss Pete Kostelnik in there also with his Badwater performance, Cross Continent , and Ke2Key. I am sure there are some fell runners who should get a shout out. When it comes to GOATs in this discipline you need to look at people who changed the way the sport was done. Style of racing or approach to crews or philosophy/ethos. Like Wanjiru or Radcliffe in the marathon, a guy like Walmsley or gal like Camille Herron in ultra running kind of redefined how top end competitors approach racing an ultra.
Best/most competitive races- Western States, UTMB (and associated events), Leadville, Hardrock, I am not super in tune outside of these. There are two handfuls of races that people with even tangential knowledge of ultra running know about. But there is such a huge range in the style of ultra running. Running 24 hours on a track at Desert Solstice is no less or more amazing than completing the Ultra Grand Slam. Running a rails to trails 100 at some stupid fast pace is no less or more amazing than grinding through the mud at Big Horn.
Bucket list- There are races that EVERYONE (or at list lots and lots of people) want to start. The big production events are there for a reason. Lotteries make this really interesting. We want what is hard to get. That is a self-fulfilling prophecy. Once something is sold out, the hype builds and we want it more. Great problem to have, but it stacks the odds of getting a given race really slim. I've got a buddy who has tried to get into Hardrock for years. He is finally waitlisted and based on recent years, should get in. It makes it really tough. So people find other events. There are some really great ones out there. But I am looking for a course that is unique, has some sort of physical character to it (mountains, canyons, whatever), is close to a place my family and I could travel to, is well directed/has a good reputation.
There is also room here for a conversation about FKT routes. Though maybe better designated as a separate thread, some of the best running out there is not even associated with a formal race. AT, PCT, R2R2R, CDT, Colorado Trail, and many, many other completely arbitrary routes that people thought would be a great challenge. This is actually kind of a neat ethos and would be a cool conversation.
A guy that transcends generations thus far is Karl Meltzer. Is he lining up to win UTMB or Western States at 51? No, but he's been running 100 milers since the late 90s, set the App Trail record (now broken) and has remained relatively healthy, with a few 100s on calendar this year, and I believe already ran at least 1 this year. I am curious to see who from this generation will be a Meltzer type 20 years from now, with no signs of stopping. He's truly a unique specimen and who's spot is cemented in ultra lore forever, in my humble opinion.
Pete Kostelnik is a wild story too. So far off the radar that people may forget how truly insane his feats have been. He made that trek from Alaska to the Keys sound wayyy too nonchalant lol.