The best nectar for hummingbirds is NOT the kind you can buy from a store, but the kind you can easily make at home. Many store bought types of nectar have food coloring or preservatives added, which can be harmful to the hummingbirds.
All you need is some plain white granulated sugar and water. You mix this in the ratio of 1 part sugar to 4 parts water. So, for example, you would dissolve 1 cup of sugar in 4 cups of water. If you wouldn't drink your own tap water, then you may want to boil your water first (which can also help to dissolve the sugar faster), and then let it cool fully before putting it in your feeders.
You never want to add any kind of food coloring! Just use red feeders, and the hummingbirds will find it eventually. Also, you do NOT want to use raw or organic sugar, as those have extra minerals in them, such as Iron, that can actually be harmful to the hummingbirds. This is the one time that organic is not better, as you want the processed sugar that has nothing but pure sugar in it. Refined pure cane sugar is the best.
Here's a link to the type of sugar I use, but you can use any brand:
It's also OK to use a 3 parts water to 1 part sugar ratio for short periods of time. Sometimes the sweeter blend can help attract hummingbirds, if you're having trouble getting them to the feeder. It also can be good in the colder winter months, if you have hummingbirds that stay in your area all year, as the higher concentration of sugar lowers the freezing point and can give them a little extra energy when it's cold. But, you don't want to do that for long periods of time, as the 4 to 1 blend is the closest to the nectar they feed on from flowers.