Can I Flash Nexus Factory Image to any Tablet? There are several different versions of Android ROMs on the Internet.

I found that no one is better than Nexus Android which is from Google.

There is a webpage on Google hosting every version of Nexus Android.

https://developers.google.com/android/nexus/images

Is it possible to flash Nexus images into other tablets like ASUS Transformer or Samsung Galaxy?
No. ROMs are always device specific. Reasons are proprietary drivers for specific hardware components.

For more details, you might wish to follow up to the rom tag-wiki, rom-flashing tag-wiki, and the sites/pages linked from there, e.g. Is there an official Android “Google” ROM?