Best:
The Embark platform is my favorite! Next, ApplyYourself, but it falls behind because of the random username they give you for your login. My favorite school specific platform is definitely Wisconsin's! So easy, especially if you're applying to multiple programs (I'm not, but I could tell). Cheaper app fee ($56), easy navigation, no annoying extras... The trick is, you have to pay extra close attention to program-specific requirements.
Worst:
I hated Michigan's! It was pretty simple, but you had to page through the entire thing (much like it seems Stanford's must have been). I thought Indiana's was great before I realized it was one of my only ones that emailed the recommendation links after you submit your app.