My visit this past May was probably the worst I have ever had. Two words: Grad Nights.
Apart from that I don't think I can pick an absolute best visit. My most recent trips as a teenager/young adult are probably the best for me because I remember all of them. I went in 2005 on a school trip, in 2006 with my mom, in 2008 during Thanksgiving with my parents, in March 2012 with my best friend, and a trip last October for my birthday. I loved Halloweentime.
All the times before then I was with the whole family and my siblings always made things difficult. Being there for the opening of Indiana Jones was cool, though. My parents took us to celebrate their 25th anniversary.