PC^2. It's very easy to set up and it's used in all ACM-ICPC format contests in my country.

CMS is being used for the IOI since 2012. It's not exactly "easy" to setup (being a distributed software), but it's well documented.

We used PC^2 in our university's last contest. it was very good and easy to learn. Competition done perfect but you must test systems and learn how to use application.

first we create a test competition with 6 participant for checking systems and it becames very important because we found some problems . I want to note somethings that we learnt :

• the documentation must be read very carefully ( we had multi operating systems for judges,admins .. and it cause some problems for us but they were our mistakes and hopefully we fixed it before contest. So I suggest you to have same operating systems,file paths,compilers,... )

• never touch computers that you selected for judges in competition time and use admin computers for checking answers and verdicts. ( because if server send submission to one of the judges and judge program was blocked, then submission will remain in Running status and that judge system will remain busy while that is doing nothing! we couldn't find any solution for that! )

I've used multiple judging systems and my favourite is by far DOMjudge (https://www.domjudge.org/). It is very reliable. It is used in several regional contests and is used as the shadow (backup) system for ICPC World Finals. Its biggest downfall is that it is non-trivial to set up (if you know what you're doing, it only takes about 20 minutes, but the first time you do it, it can easily take an afternoon). Some of their documentation is slightly behind, but if you send a question to their mailing list, you get a response quite quickly. Once it is set up, it is very worth it--we have been using it for a few years now. So if you want just a one-off contest, DOMjudge might not be the best system, but if you're planning on holding multiple contests, then you should consider it.