For textures Photoshop is the (commercial) way to go. GIMP is a good free alternative.
For meshes there are various programs. Beth games were made with 3ds max and probably zBrush, although you only need the first one if you are not a pro. Generally 3ds max can be used for most games as it is very popular. Note that there are various versions of the program though, the official MW exporter will only work with 3ds max 4 for example while the Oblivion exporters will only work with other versions in between the latest and old versions (can't remember, but it was made with 3ds max 8 in mind, will not work with the very latest 3ds max). There are other alternatives like Maya, but for Beth games 3ds max is best...except for
...maybe Blender, which has very good support (for Oblivion at least) and is free. The interface is a bit weird at first, but it seems you can get used to it.