Kali Linux is the best choice for almost every (ethical) hacker. Kali is a toolkit overloaded with advanced tools for penetration testing and digital forensics. These include tools for information gathering, social engineering, sniffing and spoofing, password cracking, vulnerability analysis, wireless network attacks and much more. Since the American TV series Mr. Robot aired, the popularity of Kali Linux has grown enormously. The show included John the Ripper, Metasploit, nmap and Shellshock.
Offensive Security, the publisher of Kali Linux offers ethical hackers the opportunity to get certifications including: OSWE, OSCP, OSCE, OSWP, OSEE and KLCP. For a fee, students are given access to teaching materials, practical examples and the exam. Kali Linux can be used as live OS on a USB stick, can be used as VM with e.g. Virtual Box and is available in 32, 64-bit and ARM version for laptops, desktops and ARM devices. Even if you are an inexperienced Linux or Kali user you can find plenty of documentation in their own Wiki, including a comprehensive installation manual and information about using various security tools in Kali.