We have been three times in October, twice in May, twice in the summer (July/August and August/September) and once at Christmas.
October is definitely our favourite time (with May a close second). Great weather - hot with hardly any rain; low crowds; Halloween Horror Nights at USF (particularly the Bill and Ted show); and my favourite thing of all in Orlando, bar none, the International Food and Wine Festival.
Both times we went during the summer we lucked out with the weather. We had really bad thunder storms most evenings (all evening) at the end of August/beginning of September and when we went July/August it rained solidly for our last 3 days. If it was August or nothing, then it would be a no-brainer, but given a choice, I would choose October.