It is tough to pick a time when the weather will be "perfect".
(was in Anaheim this June and the weather was..."perfect" high 70's and sunny)
I live in Chicago which has notoriously bad weather, so I am always prepared for everything!
Personally I like the hot humid weather and felt like I was missing it when I visited CA...
I have been to the World every month of the year...multiple times, Have been in Late Sep/Oct and it was in the 90s...have been in November when it has been really chilly.
One December we visited and it was in the high 20's low 30's at night, I have pictures of my family bundled up (kids had snow pants on) on Splash Mountain..we were the only crazy people riding, they let us stay on a for a few rides.
Last year marathon weekend it was upper 80's (not the best running weather...) so even I (who hates to swim) went swimming...it was really warm!
The January prior we were there for my little one's 2 nd b-day and it was mid 80's perfect! They swam, I sat by the pool.
My suggestion would be fall early winter to avoid the rain...easier to "predict" rain patterns than temperature.
Good Luck to you!!