If you're going for easy, I can't really help you. But, if you want to get one that would let you work on just about anything, get Sun Solaris certification. While Unix versions are different, they do have enough similarities that someone who knows AIX or HP-UX could operate a Solaris box. Someone who is good at Linux could also operate, at least on a basic level, the other unix versions. Sun seems to be pretty universally known by hiring types. Whereas if you went to apply for a job, and said you had Sair Linux certification, they might not know what that is.