Applesoft BASIC is built into every Apple IIGS ROM. It is very simple to learn, but it is not suitable for modern programming and does not take advantages of Apple IIGS capability. It does not come with an editor.
MD-BASIC is a preprocessor for Applesoft BASIC. It works only on Apple IIGS and comes with an excellent editor. The BASIC language it presented is very good, but it does not generate Apple IIGS code and suffers the same issues as Applesoft BASIC.
Complete Pascal is the best solution for Apple IIGS. It has a modern language syntax, support Apple IIGS features, and best of all, it allows programmer to paint to the full screen with minimum knowledge of the toolbox and event driven programming.