The National Gallery in Trafalgar Square is usually on the agenda of every visitor to London. A very good idea is to manage your visit/visits so you don't suffer visual overload. There is just so much to see, and it is tempting to just start in the first gallery and then find you are exhausted before you get to see some things you had at the top of your list.
My suggestion is to plan ahead. Pick up a map at the entrance, and choose the galleries that hold the works you most want to see, then make a beeline for it.
Me? I visit often, and choose just two or three works I really want to look at, one more gallery I want to spend some time in, and then have a sit down. For me my favourite painting is Holbein the Younger's Christina of Denmark, Dutchess of Milan (1538) Here you see the full length portrait of a young woman in mourning. Holbein was painting this for that old rascal Henry VIII – as there was thought he would marry her. What a fate she escaped.
Most times I wander into the Impressionist gallery and choose a different painting each time to study. You will have your own artists, artistic period and special favourites. What lucky people we are to have access to this wonderful gallery, set in such an iconic place and full of such treasures.