You will get a range of answers to this question if you are talking about picture editing software because they each have their pros and cons. Those who are happy, are happy because the cons of their software does not affect their particular method of filmmaking. For example, if you work with others in post, say you hand your audio post over to a sound designer/editor, then AVID is the obvious choice. In my experience, Vegas would be the worst choice in this scenario (although FCPX is only better in this respect with the addition of some quite expensive 3rd party software). If on the other hand ease of use and cost is more important than collaborating with others, Vegas might well be the best choice. For a more powerful combined package, Adobe could be the better choice but again, it doesn't play as well with non Adobe collaborators as AVID, nor is it as widely used commercially.
On the audio editing/recording/mixing side, the choice is much simpler. ProTools dominates both the professional and semi-pro/aspiring audio post market and as Alcove implied, is a bit of a no-brainer. Audition is very limited/weak in comparison to ProTools and you won't find any audio pros who use it but Audition might make sense for the DIY filmmaker who uses Premiere for picture editing. If a very restricted budget is the primary concern, then Reaper would likely be the best choice.
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