SAFD: Great question!
Up until this last year, I would have answered with Christmas time. In December, I got to check off a a big Disney bucket list item for myself and go to WDW for a holiday weekend with my 11-year-old daughter. I've always wanted to see all of the decorations, snow on Main St. USA, holiday overlays on rides, etc.
The trip was a bit of a mixed bag. We had very low crowds on our Magic Kingdom day, so we were walking right onto most rides. We did Tiana's four times and Tron for the first time, among others. That night we had tickets to the Christmas Party, but there were torrential downpours. I also didn't realize that while you could enter the park for the party at 4, the event itself (including special menu items) didn't start until 7. We did get to do Jingle Cruise, but mostly got absolutely drenched. We bailed at 8 and missed the parade and fireworks, but also what sounds like an absolutely chaotic exit from MK at the end of the night.
The next day we did Epcot and the weather was much more cooperative. It was a really great day, but Living with the Land was down, so we missed that, which was disappointing. We did get some holiday treats and stayed up for the fireworks, which was really nice.
All in all, it was a really nice, memorable one-on-one trip with my daughter. So, I'm grateful.
This is a really long way of saying my current answer is Halloween at Disney would be the time I'd most like to visit now.