There isn't really a "right" answer to this question. You have 3 options: lisp, dotnet, arx (in order of complexity and power). It depends on the custom codes you'll be creating, some might be impossible to do purely in lisp, a smaller subset may be difficult (if not impossible) in dotnet.
For me, I'd suggest using at least lisp and dotnet. You'd probably need at least a grasp on some basic lisp, even if you make your customizations in dotnet or arx. Even just to use as a way to load your ARX or DLL files.
If you go the dotnet route, you have a further decision to make. What language? VB / C# / other. Basically here I'd look at one where you can get the most samples. For that reason I like C#