Edler - absolute #1 power-play option on a very good offensive team. People love to overlook this guy, but he's in his prime and he's good. Simple as that.
Shattenkirk - really good first two years, but the Blues really don't score a ton
Carlson - huge upside, but not the #1 option.
However, as mentioned above by someone, Edler's bad back has to be a bit of a concern. If it's enough of a concern for ya, Shatty is a really nice second option..and Carlson isn't far off either.
But when push comes to shove, I'm taking Edler.