For now, REAPER for Linux over Ardour. REAPER is cheap, and while it is absolutely not free software, it is about as close as you can get while still being proprietary. You can use the trial for as long as you want without paying, and other than a nag screen, it is fully functional. You can rewrite some of the built-in effects, and there are several options for writing your own audio plugins and extensions.

Frankly...I vibe with REAPER, and I don't vibe (yet) with Ardour. I'm still reading the manual, and I'm still going to try keep trying it out, but there are a couple choices REAPER made that I prefer. For example, REAPER doesn't distinguish between MIDI and Audio tracks. This is really useful to write lines in MIDI before I know how to play them on a real instrument, then seamlessly use the original signal chain after the MIDI instrument. According to what I've read and worked with so far, Ardour has a few different track types.

I've been using REAPER for several years. It's been rock solid, it has all the options I ever needed, and Cockos has stayed out of my way as I transferred my license to almost a dozen computers. I wish they would open-source the software, but it's one of the few software purchases I don't regret.

What I need to clarify is that it is good in spite of its proprietary-ness, not because of it!