it's libre and gratis, if that's not something that bothers you (and to be sure if you're just writing code for your own edification it's not a practical problem) then i would recommend dyalog, although as a side effect of being open J has a (perhaps only slightly) larger free ecosystem and community.
the down side is that subjectively some people (myself included) find J uglier, less effective as a tool of thought and due to some syntactic features (hooks and forks, but no proper lightweight anonymous function syntax)-prone to people posting impenetrably dense code as "examples"
the down side is that subjectively some people (myself included) find J uglier, less effective as a tool of thought and due to some syntactic features (hooks and forks, but no proper lightweight anonymous function syntax)-prone to people posting impenetrably dense code as "examples"