I think it depends on the dynamics of the group you are bringing. While lots of people recommend MK as "most Disney", and I'd agree, but I agree more with the folks who said that the other parks can't live up to MK in terms of things to do and see. If your group includes kids that are going to be all about rides, definitely start anywhere but MK. If it's adults who'd be more inclined to enjoy the atmosphere and food and beverages offered at other parks, by all means start anywhere.
My kids and I went on a "first Disney trip" with my friend and her two kids this summer. While we did begin our park visits with AK, because I wanted her to do a character meal as her first official Disney trip kick off, we did hop over to MK for the evening. Her kids loved it! The other parks just did not hold their interest the way MK did. My kids are teens and have been many times so they find something enjoyable about each park. But in terms of things to constantly do to keep younger kids happy, MK is it. If I'd have thought about it, I would have held off on MK til our 3rd day and hopped anywhere else on that first day. But I really thought her son, who's a big Star Wars fan, would have loved HS enough to stay there all day. It turned out to be the park he complained at the most and just wanted to go back to MK for the better rides. If he hadn't have been to MK first, he probably would have enjoyed it more.
regards