We go every year in March. It is busy, but not bad. The only time we ran into big crowd was the one year that Easter was at the end of March. We went the week before, and never again! We were in MK one day, and you could barely move in the area where Splash Mt was, and the wait was 2 hrs. I told DH " We are out of here!" - which is not me!
On the up side, the weather has never disappointed us. It's been great and never humid like later in the year.