hm~ i don't think there's any specific race actually...
i'm the kind of person who falls into rabbit holes easily, i'm hungry for knowledge, especially when it connects to other things. so uma was a dangerous thing for me to get into... i started by learning about my favorites, then i got interested in the related horses (whether in uma or not). it's not difficult because keiba has many interesting stories and anecdotes. slowly you start to form fuller and fuller images of what was happening, the pedigrees, the people behind the horses, all that. though i do think starting to follow current day happenings is its own hurdle that i overcame with slightly more difficulty
it's not only that it makes uma more fun to notice all the granular detail they put into it, but keiba itself also has such a rich history, i can't help but want to learn more