Top Ruby gems for authentication & authorization are: Devise, OmniAuth, Clearance, Authlogic, Rodauth, Sorcery, Pundit, CanCanCan, Action Policy, and Rolify. Each gem has its own strengths and weaknesses, and the choice of which one to use depends on the specific needs of your application. Some gems, like Devise, offer comprehensive authentication solutions with a wide range of built-in modules, while others, like OmniAuth, provide flexible and customizable authentication systems that can be integrated with third-party services. Clearance and Authlogic are good choices for simple authentication systems, while Rodauth offers a high-performance solution with advanced features like multi-factor authentication. Sorcery is a lightweight option despite being unmaintained, but it's best suited for small-scale applications. Pundit and CanCanCan provide robust solutions for policy-based authorization, while Rolify excels at managing complex role structures. Many applications use Devise for authentication and pair it with either Pundit or CanCanCan for authorization. Additionally, gems like JWT, Secure Headers, Brakeman, Rack::Attack, Figaro, and Bullet can help secure your application by encoding JSON Web Tokens, setting security headers, detecting vulnerabilities, rate limiting abusive requests, managing configuration settings, and optimizing database queries, respectively.