Well, that's a tough call. It all depends on what you need out of the program, the price range you can afford and the wokflow you prefer.  In order of price (at least, what I'm aware of):

Mudbox has been in use in production studios for over a year and a half. It's just recently been released to the public and while you have to have a base model available before being able to use it, it offers what is being called a more user friendly interface than ZBrush.  Local subdivision is a big plus.

ZBrush is an old warhorse, who has been around for a long time and has a large and relatively friendly user base. As a bonus, you can actually start and end in ZBrush if you're skilled enough with ZSpheres.

Hexagon 2 puts sculpting and 3D texture painting in the hands of those on a budget. It's interface could be considered a plus or a minus depending on your workflow habits and the inability to export texture or displacement paint information across UV seams is a big negative that needs to be fixed. Big draw is the ability to paint textures as well as displacement but again, clean up will be needed.

Silo 2 (Beta) Like Hexagon 2, Silo 2 is a budget priced modeler that offers digital sculpting and a completely customizable interface (right down to the graphics for the buttons  you use). The open and "clean" interface can make the program a joy for some and a mystery for others. Silo 2 is currently in Public Beta Testing and can only be had at the moment by purchasing Silo 1.42 or if you're a pre-existing user.  One major feature Silo has that's lacking in Hexagon is the ability to alter your geometry without loosing all of your displacement. Currently Silo 2 can only export Normal Maps, though the final will be able to export both Normal Maps and Displacement Maps.

BlackSmith 3D is a program I completely forgot to mention, sorry about that. It's sculpting tool (available as standalone or part of the "Suite") is a pretty cool alternative and offers features akin to those of Silo 2 and Hexagon 2. I've found the interface a little counter intuitve at times and the displacement tools can be a bit tricky to master.

Curvy3D is a fun program but lacks many of the important features needed to be used seriously as a sculpting tool.  Also, it's interface can take some getting used too.

Sharp Construct is a powerful little freeware app that again, lacks some of the tools/features of the bigger boys but at the same time is free and pretty "jump in and play" in terms of use.  Downsides can be frequent crashes and polygon limits.

I've heard word that Blender also has some type of sculpting feature but I couldn't confirm this nor have I used it (obviously), so I can't comment on how well I think it's implemented or what I see to be its strengths/weaknesses.

I hope that helped.

My personal opinion is to find out if a demo is available and try the program before you buy. I love Silo 2 but I'm also looking to pick up Mudbox, as it's been getting rave reviews from just about everyone who comes in contact with it. I have used ZBrush and while I could get things done with it, there were times I felt like I was fighting the interface to do what I wanted. Hexagon 2 was my first introduction into the world of displacement painting (I'm even the mod for that forum here) but I'm not a fan of the "Validate" command you have to go through in order for some tasks to be made final, I don't know of any other program that requires you to validate as you're doing basic modeling things. Also the texture painting isn't undoable and I have yet to see movement from Daz on updates beyond 2.1 which was released earlier this year. The other tools mentioned I haven't been able to get into at all as a user really, they just didn't do it for me.  Again, that's a personal and very biased viewpoint, as it's mine and mine alone. Each user will have different experiences.