Debian Sid and Arch for most of my workstations and Debian stable on the servers.
In terms of security, the state of Linux could be better. There are steps you can take to improve the overall security of a Linux installation, some of which are easy to accomplish (enabling a firewall) but others can require a fair bit of technical knowledge depending on the distribution (full disk encryption, secure boot, SELinux, etc.).
I generally recommend that most Linux newcomers should start with Fedora or Ubuntu. They are excellent introductory Linux distributions and both offer an abundance of technical support.