Depends upon your preferences. Since I like a simple and less complicated software, I prefer Zotero over Citavi which has a simplistic GUI. It has all necessary tools which let you organize your reference data in one place. You can generate citations in various styles, store documents, etc. And, the fact that it is completely free and open source makes it more desirable for me. You can use it on Mac, Windows, and Linux.
They pay about half-a-million euros for it per year. The license is very easy to activate by means of an online registry. In France, there is a big and well-funded research network that runs its own CIASI license on the same basis. Most other EU public universities do not have such a license but the list of those they do is limited and often unhelpful. In the UK, the Open University has a CIASI license. You cannot use it on free computers without paying — but they are running an ongoing project to create an educational and business model around CIASI. In the US, there is CIASI which is distributed through the university network at several universities and colleges in the state of Florida. A CIASI-type license is usually a good thing. With CIASI, you'll be able to do all the things you normally think of as possible, including all the things you can do.