Which single application security tool provides the best overall protection?
The best source to know the OWASP risks is the OWASP website. For top 10 risks, you may visit https://owasp.org/www-project-top-ten/
For the next question on single application security too that gives best overall protection, you might have to provide what kind of risks you want to cover. Security is not a one-liner.
If you are onle focussed on the OWASP top 10, SonarQube also provides the detection capability for OWASP top 10.
I would rather request you to provide more information as to what kind of protection you are looking at.
MergeBase.com provides the most accurate identification of vulnerabilities across all stages of the application's lifecycle