adword tool

There's no single answer for your question. They each have their advantages, so it depends entirely on what you are trying to accomplish.

Briefly stated, "broad match" on a given phrase will make it eligible to appear in the results of more user searches, "phrase match" will narrow the impressions to only those searches including the complete phrase, and "exact match" will only appear when the user search includes just that: your exact phrase and nothing else.

You have to decide how each of these options fits into your overall plan, especially in terms of how carefully you are targeting your audience. That said, I wouldn't recommend using broad match to start off with, as it is difficult enough trying to figure out when your ads will appear using Google's algorithmic voodoo without running untargeted campaigns. It can also cost you a lot of money.

If you haven't already, it may help you to read Google's own explanation of the relative use of each here: What are keyword matching options? - AdWords Help. Also, if you do a general search on the phrase "Adwords strategies" you'll find all kinds of helpful information for newcomers on the subject.

Cheers,
Neil