I would hve to say top ten, ONLY because of the high tech capabilities of both Everest and Space.
I would think, as posted above, The opening of both Disneyland and World are obviously more exciting, and also the opening of each new park thereafter. I would rank the waterparks up there, and then the expansion of the monorail to MGM and AK, oh wait, that didn't happen did it. Tower of Terror was HUGE, too.
But, yeah, it's gotta be pretty high. What makes the news more exciting is that we have these great photos to look at. But I'm really excited about the August 15th Space date. As coincidence would have it, I'll be arriving on the 22nd of August. Just in time to ride it within the first 2 weeks of opening!