TAR 5 and TAR 9 got my votes.
TAR 5 because it was my first season ever and aired when I was ten. My first leg that I vividly remember was the Russia leg with eating caviar <3, blocking shots/drinking shots <3. That leg GOT.ME.HOOKED. And then, the next week, when they went to Egypt? <33333.
As for TAR 9, despite the horrendous...everything (excluding some of the cast like Fran/Barry,, MoJo, Dave/Lori, Wanda/Desiree), the route was really cool. I mean seriously. The four countries I've always wanted to go to in one season <3 (Australia, Japan, Germany, Russia). Greece was a really neat place too and Oman was definitely really neat.