What is the very strongest hold hairspray?

There are a lot of different brands from very cheap to those available only in salons or other high-end beauty supply stores. You want to look for anything with the word "freeze" in it. This will definitely give you helmet head, but if you want your hair to stand up to high wind or humidity (or just stand up), then you want something with a freeze hold. One word of advice--cheaper sprays have more water. That means it takes a bit longer to dry and depending on your hair type and style may actually cause your hair to frizz or your style to wilt. Most salon brands are very concentrated with little if any water, so a little goes a long way.

If you re-post your question in the Beauty & Style section, you'll probably will be able to get a few brands that others swear by. Regular hairspray users tend to be very brand-loyal.