Lawrence - Atlanta
PADI OW: June 1998, San Diego, California
PADI AOW: April 2004, Utila, Honduras
PADI Nitrox: April 2004, Utila, Honduras
PADI Rescue: May 2004, Utila, Honduras
PADI Deep: August 2012, Morehead City, NC
GUE Primer: July 2013, High Springs, FL
GUE Fundamentals (Rec): May 2014, High Springs, FL
TDI Cavern: May 2015, High Springs, FL (did not complete)
To answer your specific question, I consider GUE Fundies to be my "highest level of certification" but dive operators generally take the position it's PADI Rescue.