Security

Wireless sensor network is tremendous technology and has many sub-areas for research such as:

1. WBSN ( Wireless Body Sensor Network) 

2. Sensor based intelligent transportation system

3. Industrial sensor networks

4. Underwater sensor networks

5. Security in sensor networks

And also you can merge sensor network with any other field and come with new model. Work on cross-layer protocols, etc. 

Hope this answer will help you.

Among all the cited and more existing WSN research fields what is the best ?

You said: WBSN ( Wireless Body Sensor Network), Sensor based intelligent transportation system,  Industrial sensor networks ...

But, what kind of problem is the best to be treated ?

is it coverage, synchronization, access method, security, ....????

Dear Rachid, it is depend on your background of study, almost all the layers have issues and need solutions. The most hot topics are

1. WSN------- Security, energy conservation and consumption, routing

2. ITS------ Disconnectivity, network overhead, security, etc.

3. Industrial wireless networks--------- Coverage, transmission life time, link quality, etc

So first you specify your area and your field background. 

A TON OF THANKS....