Proton VPN and Mullvad VPN are both open source, meaning their code can be publicly audited to make sure they’re upholding their standards of privacy and security. Furthermore, Proton VPN offers a free tier. These are the main 2 reasons. NordVPN only protects your privacy against other websites, not NordVPN themselves. Hope this helps! Let me know if you want more details.

Edit: Mullvad VPN can also be paid for in cash/Monero, and they don’t ask for any personal information to use it (not even a username!)