Among the APL language families I would recommend are Dyalog APL and J. J is great for some of the ideas it has implemented and to gain a flavor of Iverson's vision of APL as it evolved. Dyalog APL is, IMO, more practical, and we are quickly integrating as much of J into APL as is reasonable.

Personally, I am highly biased towards using the APL symbols, and not to get "sucked in" to the idea that using ASCII symbols is easier. I think the APL symbols provide important visual and pneumonic information that actually makes learning better over time than trying to use ASCII. There is built-in APL keyboard support on Linux, and Dyalog APL comes with a good IME for Windows and Mac, so there's no reason not to use the symbols anymore.

I don't know that you should avoid any of them, but I would encourage you to use dfns if you can. Using a lexically scoped syntax is going to give you a better idea of how "good modern" APL is written. A lot of APL's don't include the J innovations or they don't have structured programming features or lack lexical scope, which is a bummer.

So, overall, go Dyalog. I'm highly biased.