Aww man, I wish I'd popped into this conversation earlier. I'm going to take some space here to advocate my favorite software.
I use Reaper for sound editing and design and Pro Tools for mixing. I used to be a hardcore Pro Tools enthusiast, but became very disillusioned with Avid (and before that, Digidesign)'s business priorities. My two big complaints are:
1) I can't rely on Avid to make Pro Tools into the software I need, ever. They do not listen to the needs of their consumers. Not enough.
2) Pro Tools is a "no" software. Want to wire Max/MSP into a track? Sorry, can't do that. Want to use some non-Avid/Digi interface? Sorry, can't do that. What about customizing the keyboard to do what I need it to? Nope. Pro Tools is wonderful when you're using it per Avid's prescription of how you should use it and what you should want out of your DAW. That's why I still use it for mixing. But if you want to think outside of the box... outside of their box, it's unrelentingly restrictive.
Reaper is a phenomenal sound design tool that is largely overlooked. IMHO, it's partly for their price and business model. Why would you trust software that costs little over a hundred bucks to be the centerpiece of your business? Why would you trust a company that permits unlimited and uncrippled trial use of their software?
But where Pro Tools tells me "no", Reaper is a "yes" machine. There's a learning curve, and it requires a time investment, but you can transform Reaper to be the customized DAW for the way you work.
I can build macros for how I use it. For me, I've got the arrow keys hooked up to a pitch shifter! I can have a temporary FX chain on any region. I can have 192khz 24bit audio files on the same track as 22.050khz 16 bit mono files. Or midi files. Or surround files. Or mp3s. Or timecode. I can assign an LFO to ANY property of ANY plugin. I can nest tracks within one another (and do away with the archaic digital busses and AUX tracks we've carried with us from the analog years!), or send audio directly to other tracks. I can chain two machines together and have one of them handle VST processing to lighten the load of the primary. If I want, I can use any I/O I have connected to my computer (and theoretically, my network). And if I want to use one device for ins, one device for outs, and one device for MIDI, Reaper doesn't try to stop me. It warns me! But it doesn't say "no".
If I'm a developer, I can create third party plugins for Reaper!
Best of all, Cockos (and the feature-building community around them) listens! There's a part of the forum devoted to feature requests. And whatever features are most popular will make it into an update. And if it won't, they'll tell you why not.
It's not perfect, of course. I don't like Reaper for mixing and the video support needs work, but I really can't advocate this DAW enough. The learning curve is definitely steep, as you have to rewire what you expect from a DAW. But man, does it pay off.