I've worked for many years with the following unix flavor of platforms:
- Linux (RedHat/Suse/Slackware/Debian)
- SunOS/Solaris
- HP/UX
I've specialised myself the last 13 years in the Debian/Ubuntu distributions of linux as I believe these are the perfect fit for mission critical platforms. I do have experience on other linux flavors but I tend to prefer Debian-like distributions.