Will you need to work with other people? If so, Mendeley, & Zotero are the best choices. You can collaborate with EndNote, using EndNote Online to sync with the desktop version of the program, but it's a much clunkier process. Remember also that with EndNote, all collaborators need to be using the same version. If you're not, then one person should add the references using EndNote.
With Mendeley & Zotero, you can create private groups that you can share with others. In these groups, you can share PDFs & any comments that you make on them, even with people who don't have access to those particular journal articles at their home institutions. Mendeley has a limit of 25 collaborators per private group; Zotero does not have a limit.
No matter which citation management program you choose, make sure that all collaborators understand which program will be used in the final document and how citations will be handled.