There is no best software for the image analysis of nanomaterials. But, I often used Image-Pro (Media Cybernetics; commercial software) for a typical analysis of nanocrystals, e.g. particle size distribution measurement and lattice parameter analysis.
There is no simple answer to this question, as this highly depends on the features you need (2D/3D images ? color analysis ?), if you can pay for a commercial software, and if you are ready to code or not. Among existing solutions:
* ImageJ / Fiji is free and offers generic solutions with graphical interface
* OpenCV is a free library but you need to code yourself
* ImagePro, Visilog area commercial software that can also provide support
* Matlab (commercial) has a rich toolbox for image processing, but is best for prototyping
* many specialized software exist for specific purpose
Depending on image type and acquisition type, maybe it will be easier to orient you.
I would defintely recommend you visit www.MIPAR.us.
It is very powerful and intuitive 2D/3D image analysis software written by scientists/end-users and is about to be released as a free trial on December 14th. You can sign up on the site as well as submit images/datasets to test.
There are many image analysis options available, but I think you may just find MIPAR to be something special if you give it a try!