When choosing the best plagiarism checker for yourself, you'll want to start with assessing whether you want a standalone plagiarism checker or something with more features to improve writing.
You'll want to consider the tool's user interface and learning curve. The simpler it is to operate, the more efficient you can be with your checking.
If you're working alone, you may do well with a certain type of plagiarism checker, but the same tool may not be suitable for larger organizations.
You'll also want to take the plagiarism tool's pricing into consideration, check how many users it can support, and analyze how well it integrates with third-party apps.