Mine has to be Champion of the Naaru. It was my first title, and the coolest sounding. I have alot of titles (more then the average person), but I will always use that title.
I kinda wish I got Hand of A'dal though.. my guild was very close to killing Vashj and Kael'Thas (we had 5/6 hyjal on farm and some of bt), but we ended up breaking up before WOTLK.
I also like Elder and Brewmaster for my shaman.. I use Elder when I'm Ele/Resto, and Brewmaster when I'm Enhance. Can't wait for 4.3 when I will turn my weapons into the two mug weapons from Brewfest. Knew I keeping them would be a good idea.