Keeps changing...
Reaper v.
du jour
n-Track v. 5.
Tracktion v. 2.0
Adobe Audition v. 2.0
I've used n-track since I started and it's a great program. I use its plug ins even with other programs. Flavio keeps developing this thing and it rocks.
Reaper got my eye about a month ago and I'm using it these days to track with. V.5 is stable, fast and is
efficient of computer resources. Using Reaper is almost like installing a performance upgrade to the computer. I like the program a lot; it represents a similar development path as n-track has, but with a different feel and direction. Sent my money right in and am happy!
I picked up Tracktion when I got a Mackie Onyx mixer and used it exclusively for a while. It's OK and solid, but I'm finding Reaper is more intuitive.
Adobe Audition 2.0 is the sleeper of the whole market as far as I can see. While I use Reaper to track and render, I'm using AA2 for everything after that - noise reduction, spectral analysis, fixing the kid coughing in the mix, all that. This thing is incredible and seems more conducive to mastering efforts; I can move quicker with Reaper but for fine tinkering I love AA2.
You can also fix old material with Adobe and do things like capture old recordings, clean them up and archive digitally. I have a collection of original 78 RPM shellac and bakelite platters from the 20s - 40s - OKEH label blues, Tommy Dorsey, all that good stuff. I plan to use AA2 to bring those tunes into rotation around my place sooner or later. I also have a small collection of Edison hard wax cylinders and a player I remember from my childhood. I think it was my Grandfather's originally; from about 1900 or so. That's scheduled to be restored (the country's expert in Edison players is about 30 miles away) and then the music is going to get preserved.
Sometimes I think this much fun should be illegal...