ZBrush. Well, it's an amazing tool, really. But I think Blackearted is right that you should first learn a thing or two about topology, before going wild with ZBrush. My way was from Silo to ZBrush, and up to now, I did not have to forget a thing.

ZBrush is great at sculpting, can do amazing dynamic geomenties, can extract geometries from other geometries and do other total amazing stuff. Incredible versatile. But not what I would start with. 

Start with Silo, learn it and create some models. Make UVs for them. See how they look in Poser and fix any problems you find. If you are comfortable with Silo, have a look at ZBrush. 

I think you should undestand what an edge loop is, and what poles are before you should try retopology. ZBrush, Silo and a number of other apps can do retopology, but you should still know what you're doing. 

If you need a full package, go for Modo. You will most probably still use ZBrush later, but first learn the basics. All IMHO, of course.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.