Which is good to target, singular or plural keyword?

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":

• baseball bat: 18,100
• baseball bats: 18,100
• baseball hat: 2,900
• baseball hats: 6,600

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:

• Use both version in the page title if you can write it in such a way that sounds natural. Its hard to do with singular and plural though.
• Use both versions within the text of the page.
• Create several pages that explore the topic and use both versions. For example pages with titles such as:
  • BrandX Baseball Bats
  • Which BrandX Baseball bat hits the furthest?
  • BrandX Baseball Bat Reviews
  • Compare Baseball Bats: BrandX vs BrandY
  • Baseball bat weight by player age