What airfoil (or family) has the highest $C_L/C_D$ ratio at speeds in the $50\:km/h$ range.
The use case would be an ultralight with a max speed of $50\:km/h$
Please read the answer to this question first and try to answer the questions listed there. You need to provide more information!
Next, the best place to look is among glider airfoils. The Wortmann FX 63-167 is now 55 years old but still a good start. It was designed for the low speed of a human powered aircraft and has been used on gliders and motor gliders.
But the airfoil still can be improved, for example by adding flaps or by narrower tolerances for the surface contour, especially around the nose. But this will make construction more complicated, so the overall best compromise might still be the Wortmann airfoil.