What is the most important component of a successful ad?

The most important thing is to make a catchy headline for your ad.

Is the call for action attractive enough? Does it address your target audience's need? this will make your ad successful to the people you want to reach and this is how they will remember your brand as well as the ad.

For me its always good copy.
But then my views are not those of a typical target audience.
I always fall for good copy.

Most of the time, the only reason you would read a copy is because the Ad is placed and designed properly to attract a viewer.
I'm gonna have to say placement and design.

The Headline is the first part of your ad that the prospective Client , it must grab them or they will just go onto the next Ad in the section.

A radio or tv ad has to be attractive, concise and straight to the point.
Knowing your target audience well helps a lot, and makes the customized multimedia packaging of your idea more appealing and, eventually, the delivery of your message much easier.

Honesty is always the best way to touch the hearts and minds of people

The ad placement.
You can have the most compelling ad, but if your target audience never see it then it’s not going to make any difference.

The most component part is ; the focus on the Subject should be simple & minimum wordings to cover the subject.