I'm not very good at "shorter" races period. Never been tested but I'd guess my Vo2max is around 72. But please don't say I duck the competition.
I've toed the line at the USA 12km Cross Country champs this year (got destroyed), US Road marathon National champs at LA, and then did the Boston Marathon.
Then of course I also did Comrades this year 5 weeks later (which when you take the now accused drug cheats out I got 13th place). I had a rough go there and got humbled. I'd consider that to be the most competitive ultra in the world...
For short distance mountain running I did do the Pikes Peak Ascent last year (when it was the World Mtn. Long Distance Champs as well)...that is a pretty short and high-end race for me and it's hard to specifically train for.
It's really hard to be sharp and fast at 12km -20 mile mountain racing when you're training for 50km to 100 miles on all sorts of terrain (road to trails to mountains).
Then of course I'd consider the most competitive trail ultra in the US is the North Face 50-mile champs in SF (gets all the guys from WS100 who won as well as races like Lake Sonoma and other roadies who are OTQ qualifiers in the marathon). I did that last year as well.
I actually try to show up at high profile, competitive events (hence why I'm doing UTMB100) but a lot has to do with my schedule and what I think I might be able to pull off. MUT Running really takes you out of your comfort zone with all the possible variety you can experience. Doing races on different surfaces at different distances in mountains/trails/roads around the world takes time to adapt to and I'm sometimes learning that the hard way!
Best of luck to all of you in your future racing and training.