There is no best, there is only pros and cons.
You didnt say which windows. The paint program that comes with winsows might be enough for you but you would need a converter to/from the TGA format. Such as IRFANview.
http://www.irfanview.com
PhotoShop is a good program but it costs money.
GIMP (GNU Image Manipulation Program) is free, runs on Windows Linux and MAC, and its what Illwinter uses. In fact its fast becoming the program of choice for programmers in general (O'Reilly has a book on it. In my world that fact alone tells me alot). The drawbacks is that its a monster truck of a paint program. Its not the easiest one to learn. There are web sites of tutorials, and books on the subject. People LEARN Gimp like they learn a programming language.
http://www.gimp.org/