Whenever I go to local stores to buy cakes, I find they're covered with something called "whippy." I don't know what it is, but I don't like it. In the past, I also tried a boiled glaze frosting; in my opinion, it was just plain awful. Whether you have a sheet cake, layer cake, cupcakes or brownies, the best possible frosting is buttercream. It is easy to make, delicious, and can be made with number of variations.
All you need to make standard buttercream frosting is a one-pound package of confectioner's sugar, a teaspoon of vanilla, and either butter or margarine. After adding the vanilla to the sugar, add the butter or margarine gradually. You can use up to a pound to get the consistency you prefer.
Variations: if you like chocolate buttercream frosting, add three slightly-heaping tablespoons of regular Hershey's cocoa. Add a teaspoon of cinnamon if you wish.
Years ago, when I often had cakes from bakeries in New York, I noticed there was a difference between their frosting and the frosting I was familiar with. It turned out the "secret ingredient" was Crisco. I'm not joking, and it tastes much better than it sounds. If you want to try it, simply use Crisco instead of the butter or margarine.
If you want to get extra- fancy, you can use a pastry tube to make piping and flowers on your cake. Separate your frosting into however many different colors you want, and add a few drops of food coloring to each.
Warning: while you can use different flavors if you do not want plain vanilla or chocolate, I tried this years ago when I ran out of vanilla. I didn't know the lemon and orange extracts had a high alcohol content until my former landlord ate the whole bowl of frosting and became, well, snockered. So if you want lemon, orange, or another flavor, you may want to use a small amount of juice instead.