Filebeat and Beaver are the logstash agents?
"Agent" isn't a frequently used term in the Elastic community nor in the official documentation. Filebeat and Beaver are two programs for shipping log files to Logstash servers and, as such, are usually called shippers.
It's like, if I want to connect my Websphere logs to ELK, I need to install this logstash agents in all of my AIX server right?
Well, you should install something capable of shipping log files over the network. There are many options. Filebeat and Beaver were the two that popped into my mind. NXLog is another one but as I believe it's written in C or C++ I suspect it might be less portable than the other two.
and which is the best to use?
Given your choice of operating system I think you should go for anything that works.