Grill 23 in Boston
The Kobe Cap Steak here is melt-in-your-mouth delicious. I like the filet at Ruth's Chris better though (if we're talking about Boston's steakhouses).
As far as steakhouses I've eaten at while traveling, I thought both Gibson's in Chicago and Mike Shannon's in St. Louis were top notch (I generally order the filet when I visit a steakhouse for the first time).