Who Offers the Best Downloads?

Not too long ago, I purchased an Xbox 360 for my family. We purchased only a couple games upfront. We weren’t sure what the best games were or the most popular games. We want to make sure that we have the right games for our children to play. Because we want, and need, more video games, we wondered who offers the best Xbox 360 downloads. We don’t want to pay a fortune either. Is that possible? If anyone has any recommendations, we would certainly take them to heart. We are hoping to also find a site that has a wide selection of downloads. Our children are ages 6, 10 and 14 so we need a variety of options from which to choose.