How would you decide and choose the best media to promote your advertisement?

It depends upon your target audience and also the type of product your are going to advertise. For example. IF its a mobile device that has to be promoted the best place would be Television add why because the visuals matters a lot than images. Videos are even added in online retail sites such as amazon, Flipkart etc so i would suggest Television add and Online sites

As rightly conveyed by Sanjeev it depends on the target audience and the product you are trying to sell. Another factor which is important is the cost or the budget of your advertisement spend.

Lets look at the different advt media TV each ad last for10-20 sec and cost a huge sum if it needs to be advt during peak time. Newspaper ad's are location specific and their life is only for the day as the following day the same newspaper is in the trash can.

Today most of the advertisement happen in the digital media world as the target audience is global and not restricted to a country or place also with limited budget you can reach out to specific set of people based on gender, location, age, etc... which is not possible in any other media which is currently existing, in short is is target advertisement and reaching to people who would be using your product only. 

1) The age group and reach to the targeted audience.

2) Budget and time frame available.