A brief description of each (in reverse order of development from Release to Nightly) - https://www.mozilla.org/en-US/styleguide/identity/firefox/channels/

You asked your question using the current Nightly version 26.0a1.

In order of development - some new features may come and go from channel to channel.

• Nightly - Under heavy development. Least stable/secure. First tests of new changes/features; some changes/features introduced in Nightly may be removed before Release and other versions. Only for testing. Should only be used by very experienced users/testers.
• Aurora - Still under development. More stable/secure than Nightly. Some bugs may still be present that need resolution. Should be used by experienced users only who can post/report reproducible problems and work around issues.
• Beta - Final development stage before Release. Usually good stability/security. Major bugs resolved. Working out final bugs. Preview of what Release version will most likely contain in the way of changes/features, though some changes/features can still disappear before Release.
• Release - Final channel released to public for everyday use.

what is the best (firefox or nighty) ?

Firefox. It is the release version.

For the average user for daily use, Firefox (Release) version is recommended. Current Firefox Release version is 23.0.1.

First, some advice.

• Always back up your profile when changing from one channel to another.
• You should backup your profile regularly for safety sake.
• Some good information here - http://kb.mozillazine.org/Going_back_to_an_older_version

To change channels, you must download and save the current version, close the browser, run the installer, accepting the defaults.

• Download Release version - http://www.mozilla.org/en-US/firefox/all/
• Download Beta version - http://www.mozilla.org/en-US/firefox/all-beta.html
• Download Aurora version - http://www.mozilla.org/en-US/firefox/all-aurora.html

You may need to reinstall/transfer your extensions and transfer other important data (bookmarks, passwords, etc) IF a new Profile is created, or you can transfer them to your new Profile per the article below.

• Transfer important files to a new profile - http://support.mozilla.org/en-US/kb/Recovering%20important%20data%20from%20an%20old%20profile or http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
• Information about Firefox Profile - http://support.mozilla.org/en-US/kb/Profiles or http://kb.mozillazine.org/Profile_folder_-_Firefox