All 3 are equally important.

Meta Title:
Make it short, and get to the point. This is a support title tag that some search engines read, some don't. You should relate the title as close as you can to the products that you sell on your site. It should not be longer than 100 characters all together, including the white spaces between words.

Hint:
Whatever it is that you sell on your website, go to one of the major search engines like Yahoo or Google and search for products that you sell. Use search phrases and search words that you think are most likely used by Internet searchers.
Then, check the titles of the first 10 resulting websites. The title is the part that is (usually) UNDERLINED, and links to the website.

Meta Keywords:
We made it easy for you with the keywords. Just take the website address of one of the "top resuts websites" and plug it into your meta keyword section.
Our software will automatically find your competitor's keywords and "pull" them into yours.
Change them around to your likings and hit submit! You're all set!

Hint:
Don't use more than 8 or 10 keywords! Many big search engines will possibly "kick you off" if you have too many keywords in your meta tags...

Meta Description:
Use 3 or 4 phrases that describe your business and your merchandise best! Don't over-due it! Keep it simple...