What are the best-known supermarkets that are in the Netherlands, particularly in the Leiden area? I'm looking for regular grocery stores for food, bread, vegetables, shampoo.
Well, the biggest and probably most famous (in the Netherlands) is Albert Heijn (with their logo being "ah" glued together), which was established in 1887. Looking at the list on Wikipedia, and Gala's comment, I wouldn't presume to list the "top 5", but certainly in addition to Albert Heijn you have PLUS, Jumbo, Lidl, Aldi, SPAR being somewhat frequent in various regions. Most or all of these you should probably find Leiden.
Oh, it looks like there's a longer list on Dutch Wikipedia.