I can't specifically claim that the corned beef hash is great, but a trip to Goody Goody Diner is de riguer when a group of my friends gather in St. Louis. Sometimes more than one trip.
I checked the menu on their website http://www.goodygoodydiner.com/picts/... and it does list corned beef hash as an option.
I can't speak as to whether it is great or not, but I went to Carondelet Diner today (for the first time) and they had corned beef hash on the specials board. Everything on my plate was pretty damn good, so I'm guessing their hash would be, too. The chicken fried steak was just about the best I've ever had. Carondelet Diner is just a little place on the way-far South Side in the Patch neighborhood. A friend recommended them for breakfast and I owe her one. Very simple. Big portions, I'm thinking you could call them and ask when they'd be featuring the corned beef hash...(314) 875-9994. They seem to have a website, but it is wonky.