I have used several methods, they all seemed ok. Since I could only be in one park at one time I had no way to judge if I was really in the best park for that day, but I never ran into horrible crowds even though we went in May or June, both fairly busy times. The Unofficial Guide (way back, but I can't imagine it is not still good) which is the same thing as touringplans.com , the whatever park had emh (or EE) the day before, and on my last trip Tourguidemike. All three methods worked.
Hint- use the avoid this park info... so if on a given day TP/UG says avoid MK, and TGM says avoid MGM, and MK had its EMH the day before then go to Epcot or AK that day... which park is the worst is what you really want to know. Often all three methods will warn you away from the same park.