What is the most useful tool that marketers can use to develop IMC campagin?

There is no simple answer to this question. Because, answering such question need dept search on the situation of the product and the segment whom this product will serve. However, the most effective IMC is the one where all communications channels and ways are used on it. 

The IMC process generally begins with an integrated marketing communications plan describing the different types of marketing, advertising, and sales tools to be used during campaigns.
These are largely promotional tools, which include everything from search engine optimization (SEO) tactics and banner advertisements, to webinars and blogs.
Traditional marketing communication elements such as newspapers, billboards, and magazines may also be used to inform and persuade consumers.
Marketers must also decide on the appropriate combination of traditional and digital communications for their target audience to build a strong brand-consumer relationship.
Regardless of the brand’s promotional mix, it is important that marketers ensure their messaging is consistent and credible across all communication channels

It is called IMC and starts with "Integrated", then all have to be integrated to get the best results :)