I was a solo traveler in Amsterdam last October. I dined at the Pantry, the Seafood Bar (the location near Vondel Park), AMOI (Indonesian), Buffet von Odette, and Casa Peru. All were good. The first four were recommended in guidebooks or foodie blogs. At the Seafood Bar, AMOI, and Odette, as a solo diner I was able to get a seat at the bar without a reservation when the restaurants were otherwise fully booked. Casa Peru (Peruvian) was an unplanned wildcard that I ducked into during a rainstorm because there were free tables. At no point did I ever feel any issue being a solo diner anywhere in Amsterdam (or Leiden or Ghent in Belgium).