what website i use depends on what data i'm looking for, but what i post caps of on twitter is usually jbis. cuz it's what i use the most and it's easy to screenshot in a readable way (since on sns i'm trying to present information)
netkeiba is probably the best, but i don't feel like making an account (lazy lol), so i can't look at race lists and stuff. so it's easier to go on jbis, but also netkeiba has a lot of cool extra stuff like user submitted photography on each entry. (also i think it's the only site i use frequently that shows racing silks easily)
sometimes if i'm having trouble i'll use umanity, or i just plain search keywords in japanese and pick based on what i need (usually i end up on articles)
if you want information that's like, specific to a particular race, it absolutely has to be JRA most of the time (search the race name in japanese, it usually takes you to a list of years, you pick from there probably i forgot already). this is where i check the full list of what horse is in a certain race, popularity rank there, etc. ... it usually also has an embed from their youtube channel of said race if it was a graded one
but yeah what site to use is a matter of what info you need. i just find jbis very digestable and easy to use most of the time