Okay, i'm looking over all of these, and seeing my flaws- Les Mis and Phantom are two of the same. They're inevitably always going to be a part of Musical Theatre due to thier ridiculous longevity. They are not that influential in any other way though, so there definitely isn't room for both on the top ten.
Mamma Mia!, as much as I hate to say it, is a milestone of Musical Theatre. The jukebox musical is now a huge part of Musical Theatre, love it or hate it, which means it should be on the influential list.
On my Sondheims, everything should be on the influential list. Company was the first concept musical, thus deserving a spot on the list. Sweeney Todd was the first horror musical, which are becoming increasingly popular. It's yet to be seen how durable of a genre the horror musical will be, so its acceptance is really pending (I'm sure if they brought Rebecca to Broadway it would totally inhance the genre- I have mad love for Rebecca). Sondheim is just God, and everyone should bow to and worship any show he has ever made.
Gypsy is flawless, but it isn't groundbreaking, which makes it hard for me to put on the list. I agree it should be worshiped and adored, but not quite groundbreaking enough to be on the list. Someone prove me wrong, because I loooove Gypsy, but being fair, I don't think it belongs.
TBC
"There are only two worthwhile things to leave behind when we depart this world of ours: children and art."
-Sunday In The Park With George