I'm partial to Mom's Deli because I grew up in south St. Louis. But I think anyone who tries their Poorboy sandwich will agree it's one of the best deli sandwiches in town. But even if that's not what you prefer, they have an assortments of special sandwiches (Charlies, Mom's, or just good old roast beef) that will probably appeal to you.
The staff is always friendly and the prices are low. The only downside is that on Saturday and Sunday afternoons, the place is packed from about 11 am to 1:30 pm. But I guess that's the price you pay for making such excellent sandwiches.