Now I'm not the leading expert or anything but I do know what I like.  Photoshop and Corel are probably the two main ones along with illustrator.  All three are good but the drawback is that all three are super expensive.  

Photoshop is a whopping 549.99, Illustrator 573.21, and corel 395 but you do get a whole sweat of stuff for that price.

Luckily I got photoshop free cause my hubby is in school.  I have used all three programs but illustrator only once.  It was super complicated and I didn't see the point of learning it when the other two gave me the results I wanted.

There are a lot of free programs out there Gimp is supposed to be just like photoshop.  I have never tried it so I can't tell you wether that is so or not. PD Pro Digital Painter 3 Powered by Project Dogwaffle, I had never heard of this but it says it simulates real painting.  Artweaver- says it's like photoshop and painter merged but it doesn't have layers.

3d programs- do not I repeat do not pay for 3d, there are a lot of free 3d programs out there.  Daz 3d is a great site and if you want to really create 3d they also often have deals to get great 3d software for free like Bryce or Hexagon.

Google sketchup is also a great program- It is pretty easy to learn how to do the basics.  For serious 3d artists who also want to animate and do gave design Blender is a good way to go.  There are tons of tutorials and though the interface looks confusing, it isn'g so bad once you get used to it.  

I didn't mention it above but I did mention my favorite drawing program in the info on one of my pictures.  SAI, or Paint tool SAI.  It is a japanese based program and ideal for creating line art and anime and manga.  You get a 30 day free trial and after that it is only about 70bucks!  It is seriously easy to use and creates great results.