In the corporate world it's mostly Red Hat - as it's a hands on exam --> you can't do --> you can't pass.
Linux Foundation also have decent hands on Linux exams (not quite as on par / in demand as RH, but decent).
RHCSA (RH certified system admin) --> RHCE (RH certified system engineer) --> RHCA (RH certified architect).
I'm a RHCA and you'll learn a lot from RH exams.
Additionally check out some of Sander's material - like this or his RHCSA / RHCE tracks to learn more about Linux or Red Hat in general.
Over 9Hours of Video Instruction Description Learn all about Linux internals and how Linux really works. Linux Under the Hoodis a unique video project that focuses on Linux... - Selection from Linux Under the Hood [Video]
www.oreilly.com