Best forum software to convert from

I only converted an SMF to vB 4 in the past and it was not what I expected. I want to start a new forum and if I see it is growing then I'll move to xF. My question is that which forum software should I use in order to get the best results when converting?
So I can use...
  • vBulletin 4 - I have a valid license
  • SMF - I worked a lot with SMF
  • phpBB - I don't really like it but I can use it actually
I'm not sure there is such thing as "best" but if there is then I want to know :)