I think the best morning EMH is MK because of the sheer amount of rides open during EMH. We always do morning EMH at MK and we can knock Tomorrowland, Fantasyland, Liberty Square, Frontierland, and Toontown by lunchtime. Epcot is next in terms of number of rides open - Spaceship Earth and JIIF are the only 2 that we really do so it's not worth it for mornings.
We don't usually do MGM and AK mornings just because there really isn't enough rides/attractions to hold us for an hour. I mean, I'm not about to do Kali first thing in the morning.
As far as evenings go, we went when they had the trial back in May 04 and then when it was fully instituted in Jan 05. I find that evening EMHs do tend to be more crowded than mornings - that's a given - but when you're faced with an early closing park and an additional 3 hours of fun, I'll take the added 3 hours any day. After surviving Thanksgiving and New Year's Eve where the parks are packed, evening EMH doesn't feel crowded at all.
So, it's all in perspective to me.