Defining the "best" tool is a difficult thing to do, as it can be very subjective. However, there are a number of options:
1. If you have the sculpt map files already (ie you're purchasing sculpts and just want to paint on them in 3d), I'd suggest Photoshop CS4 Extended. The Extended version lets you work in 3d using with many of the same tools you already know from working in 2d. You'd need to learn how to import the sculpt into Photoshop (using some other 3d tool or plugin to convert it to an .obj file, I believe), and then you'd need to familiarize yourself with the 3d portion of Photoshop CS4E's interface, but that would let you paint in 3d using much of the knowledge you already have. There are tutorials on Adobe's website about the 3d interface, as well as some excellent info in the Content Creation/Texture forums on the main SL website.
2. If you also want to create the sculpts, you could use Blender. Pros: It's free and it allows you to paint as well as make the sculpt. Cons: I understand the interface is very different than the norm, giving it a bit of a learning curve.
Other than that, I suggest you check out the SL Wiki, and search for sculpts or sculpties. There's a page listing all of the available 3d modeling tools, their costs and a comparison that might give you a better idea on which of them would best meet your needs.