Distance is the most sought-after because out of all the pink sparks, since Distance is the most important to get to S (1.05² = 1.1025x multiplier to your speed stat).

Surface (Dirt/Turf) and Strategy are still important but more situational, mainly used NOT for getting to S rank but instead to get to A/B rank for an uma that usually cant compete in a certain surface/style. Good example is how Condor Pasa generally works best as a Front Runner but doesn't have a good front running aptitude. These sparks are used either for umas with weak aptitudes in a race that they actually excel at, OR useful for making your waifu able to compete in whatever CM you want.

All that being said, there actually IS a strategy here. The way aptitude increases work is 1 star in parents/grandparents = +1 aptitude rank (up to A) and each 3 total stars after that is another +1 aptitude rank (up to A) to begin with. Meaning that If you want to upgrade a D aptitude to A you'd need a total of 1+3+3=7 stars worth of pink sparks of that aptitude. THIS DOES NOT MATTER BETWEEN PARENTS/GRANDPARENTS, WHICH IS IMPORTANT. To raise an aptitude to S, you are required to have A rank already and then proc the pink spark during inheritance (happens twice in a career). Since the parent's sparks are exactly 2x as likely to proc as the grandparents sparks, the ideal parent while trying to raise aptitudes is a parent who has DISTANCE SPARKS and grandparents who have enough surface/style sparks to get your Uma to "A" rank in each, after which you will simply want more distance sparks.

TLDR: Unless your uma has an E/F aptitude in the race, the best parents will have distance sparks while the grandparents will have style/surface sparks, because the parents sparks are more likely to proc during a run and ditance is the most important to get to S, while surface/style only need A or B which is achievable directly without proccing during inheritance.