The problem is balancing "greatest" with "my favourites." Starlight Express is one of my favourite musicals, but I'd be damn hard pressed to put it on a top ten list when assessing shows for actual quality. Factoring in production is also difficult - where does a great or awful staging get in the way of assessing the underlying material?
West Side Story shouldn't be on the list of the best musicals of the last 30 years. I am off to see its 50th anniversary tour in 3 weeks!
I love Wicked but it probably isn't the BEST musical of the last 30 years either. As I have said before I just would not be able to pick a favourite, and people will NEVER agree on a favourite either.
ALW's shows aren't likely to come close when there's gems like Breakfast at Tiffany's and most of Frank Wildhorn's catalogue - ALW is many things but he is rarely boring, and that's what the worst shows are.