edge detection of image..

I suggest that there are different 'best fields' on imaging processing depending on the current impact of applications on society. For example: 

Image sharpening and restoration

Medical field

Remote sensing

Transmission and encoding

Machine/Robot vision

Color processing

Pattern recognition

Video processing

Microscopic Imaging

classification and segmentation 

compression - old and still very active field

superresolution - one of the hottest in last 5 years

denoising and enhancement - everyone needs this

Image classification and recognition. Image-based drone navigation. Superpixels.

Processing and denoising of medical Images.

The THZ Imaging

Sparsity image processing

Pattern recognition