Can you recommend a good CAD program?

[Jaco van der Molen]

Hi, I am a huge fan of the newer studless design, but I find it quite hard to build with it because it takes many iterations.
That's why I am currently searching for a good Lego CAD program to develop technic mocs. If found the Lego Digital Designer, LDraw and LeoCAD. I installed LDD and I think it works nice, but I'm missing some features. Since there are alternatives, I suspect that they have additional features which LDD has not.
I'm looking for the following features:

• replace parts
  
  
• modular building (import of pre-build assemblies)
  
  
• Import part lists from sets you own
  
  
• generating (and modifying) an instruction
  
  
• validation of gears and checking for degrees of freedom
  

I know there is propably no porgram which can handly all of this. That's why I want to ask people who are experienced with Lego CAD, which program they prefer and why.
Thank you in advance!

Well, Ameer, you've come to the right place!
I'd say try LDCad for modelling and LPub3D for creating instructions.
LDCad can do most features you wish. LPub3D can create instructions.