Which keyword is better for targeting and on-page optimization, singular or plural? Like "baseball bat" or "baseball bats"?
Google usually shows differing search results when people search for the singular vs the plural. So SEO has to be done separately for the different terms.
Start by figuring out which version (singular or plural) has the higher search volume. Use the Adwords Keyword Tool to compare search volumes. Make sure to select "[Exact]" as the "Match Type" in the left hand column. Doing so, I find the global monthly searches for your example and "baseball hat":
So for "bats" target both the singular and the plural, for the "hats" start with the plural and consider the singular secondly.
To target multiple keywords:
Singular keyword content examples:
Plural keyword content examples:
I do recommend to conduct some analysis for this. Keywords with most traffic doesn't always mean that there is gold. Get what I mean?
OK here we go - example only:
Perhaps, an experiment would probably a good idea buddy.
So, to answer the question above, you must conduct a test for you to answer your own question.