I've watched the past 4 Hakone Ekidens online after coming across the amazing duel between Murasawa and Ugachi in the National University Ekiden. I was blown away by the passion of the runners, the crowd support and the production values of the broadcast. I've been hooked ever since and have made it a tradition of watching Hakone every year and I'll watch the other Ekidens if I have a chance as well.
I'm not sure it's the #1 sporting event in Japan. Sumo and Baseball probably get more coverage. I remember Brett mentioning a while ago that it gets about 30% viewership in Japan with over 1 million spectators on the course so its definitely a premiere sporting event in Japan though.
One of the cool things about the race is that it takes around 12 hours to complete over 2 days and there is always something exciting happening. With the race being a relay with varying lengths and terrains, there is a lot of strategy in where the Universities place their top runners. Some try to frontload the race and then hold on in the later stages while others try to move through the field. Because of this, there is always movement in the race even though they are running long stages in a very long relay. I find it more exciting than NCAAs in cross country and it is over many hours and 2 days, not just 2 races in 90 min.
There are also several exciting things for the viewer outside of the overall competition. Each stage has a long history behind it and stage records are a very big deal at this point. In addition, the top 10 teams automatically qualify for the competition the next year and there is a great prestige in making the "seeded" section. Also, if a team falls over an accumulated 20 minutes behind, their next stage runner has to start the stage without his relay partner handing off their team sash. This is a huge deal for the back teams that are either having off days or barely qualified for the competition. One of the most dramatic scenes of the last few years was a runner that fell several times in an attempt to get the sash to his teammate before the cutoff. He managed it with 1 second to spare.
Brett probably knows best what races most of the top teams do but to my knowledge these are the competitions I see results for most top programs.
October - Izumo Ekiden(shorter stages but first really meaningful race)
October - Hakone Yosenkai. This is a 20k road race that the teams that aren't automatically qualified compete in to see who fills the rest of the field. The total time of the top 10 runners is used as the deciding factor.
Early November - National University Ekiden(this is actually their national championships but since basically all the top programs are in the Kanto region, Hakone is better and has a longer history). Stages are a bit longer
Late November - Ageo City Half Marathon. I think this race is used by a lot of Universities to decide their Hakone squad.
December - Nittai University Time Trials. Run some fast 5k and 10k sharpening PRs before the big event.
January 1,2 - Hakone Ekiden
For predictions. I'll go with Komazawa, this looks like their year as they are deeper than anyone though I wouldn't rule out Toyo, Waseda or defending champ Nittai. I'd like to see Meiji and Aoyama Gakuin mix it up as well.