[1llusion]

Well, every language has its pros and cons. Python is widely used for writing PoC as it is crossplatform. However, you can't write good malware with it

Java is popular for crossplatform malware. But you can't do really targeted malware.

C is probably the most popular as it is a good compromise between targeting, speed and complexity.
ASM is popular for highly targeted malware (Stuxnet-like stuff)

It all depends on what you want to accomplish

Don't take .net!! Too many dependencies! Take Delphi. It's one of the most used blackhat malware writting language.

Where did you take that from? O.o

Stuxnet - ASM/C
SpyEye - ASM
Zeus - ASM
StealthBanker - ASM/C

You have Cybergate from Delphi family, but Delphi is far from being the most used.