What is the best data classification tool for a mid-size business of 2300 employees? Need a cost-effective tool that is good.

You may want to consider the location of your data (on-prem, cloud, hybrid), type of data sources and any other specific functionality that you need for your organization, for data classification. Based on this, you can evaluate the various tools in the market. Selecting a tool first may impose constraints of the tool on your data classification abilities. Hope this helps.

We're still in middle of implementation steps, but Microsoft Purview seem to do a great job. Depending on what type of MS licenses you have, this may already be included in your licenses (e.g. E5). Whatever you install, be ready for a long project, since you may need HR and Legal alignment, and also the machine-driven categories will need quite a bit of supervision.
 
URL: https://learn.microsoft.com/en-us/purview/purview