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: