That's really a matter of personal preference. They all have strengths and weaknesses and their performance depends alot upon the skills of the player and his character.
Personally, I use the MP5. I forget if Vegas 2 has the 10mm variant or not, but on previous games that's what I always used because if you read the book it's all they ever used.
ahh.,
but in the assault rifles.,
what do you most use?