Personally, I have never experienced anything like the crowds at the Magic Kingdom at Universal Studios. For those of us who stay onsite, it's hard to make a comparison because we are going to the FOTL on all the rides at US. However, I still say that the crowds at MK are the worst. There are times at MK when you can't even walk. I have not experienced anything like that at Universal. To be fair, they are really quite different. A better comparison is MGM to Universal. They are similar parks because of the movie theming.
Just my two cents, of course. I'm sure others will share their experiences with you.