We have studied random sequential adsorption (RSA) of three classes of polygons with rounded corners: rectangles, isosceles triangles and orthogonal triangles. Using the algorithm that allows generating strictly saturated random packing, we have systematically determined the mean saturated packing fraction for RSA configurations built by these shapes. The main aim was to find out the figure that forms the densest random configuration. Although for rounded rectangles the packing fractions were lower than for discorectangles, the densities reached for some rounded isosceles and right triangles exceed the highest known two-dimensional packing fraction for configurations built of unoriented monodispersive objects. The microstructural properties of several packings were discussed in terms of the two-point density autocorrelation function.