I have run an in home daycare for several years now and I made a list of the most popular toys... some of them might surprise you.
The most popular toy I have for all ages and both genders has been the toy stollers ($4-$8 at walmart). My kiddos fill them up with everything and parade around the house in them.
The toy kitchen, the dress up box (halloween sales and thrift stores can help you fill this or even just various pieces of cloth, so they can design their own clothes) and toy phones.
For the babies and toddlers, I have a Fisher Price Incredi-Block and even my 4 year olds still play with it. You can find them on ebay and craigslist, but I found them to to be cheaper new from the FP website once you added shipping costs.
And just a sidenote, I would suggest taking some of their older toys and storing them for a few months. Then rotate them out with some of the new toys they got more recently. It gives them variety and "new toys" without the added cost. We just rearranged my 3yr old son's room and put his toys in new places and rotated out some older toys. Now he is thrilled again with all his "new" stuff.