The most famous sight in Mostar is the Stari Most, the Turkish Bridge, crossing the River Neretva. Although destroyed in 1993, the bridge was reconstructed as faithfully as possible to the original specifications. Today, this bridge is the main reason most tourists come to Mostar and there is a long debate as to where the best viewing place is. The problem is that the bridge faces due north or south depending on your position and so, during the day,the bridge is best seen from the south side during the late morning and early afternoon, when the sun is at its harshest.
My suggestion for the finest view is whilst having dinner on the terrace of the Urban Grill restaurant. Sit at the table to the extreme right as you enter the terrace and the bridge will gradually start to glow as the sun goes down, due to the tasteful night-time lighting. Sitting at this table aligns you with the exact centre of the bridge and so pictures achieve an almost perfect symmetry with reflections in the water.
An added bonus is the call of the muezzin at sunset. This call echoes from two or three minarets around the city and reverberates around the surrounding hills. When I was there, I also heard the quacking of some ducks who were upset when a diver jumped into the river from the bridge 21 metres above. Sounds travel a long way in this beautiful setting.