It really depends on your goals and skill levels.
What do you want to do with it? Are you going to release it? Is the source to be private or public? Is it to be server-side or client-side?
There's 4 choices from my 15 seconds of thought on the question.
1) C#
2) Java
3) Python
4) PHP
Of these each has advantages and dis-advantages.
1) C#
+ Looks nice
- Windows Only
- Can be decompiled if not obfuscated well enough
2) Java
+ Works on all OS
- GUI can be a PITA
- Can be decompiled
- GUI feels slow on windows
3) Python (I've not used)
+ Works on all OS
- Can be decompiled (I hear)
? GUI.... ?
+ I've heard good things about it
4) PHP
+ No type casting required
+ Works on any OS
+ Code can be easily ported into any PHP website
+ Does not need to be compiled
- Has no GUI
- Has no threading