We flew a few segments on CO last year (all in F except one ERJ) and I would mark it a toss-up between CO and NW for best F just based on our experience, but we mostly flew lunch rather than dinner flights, so making direct meal comparisons is difficult. CO's service was fine, but again we have limited experience. The plonk is at least drinkable and sometimes actually pleasant on CO and NW - I can't say that was the case on DL. The best FAs on NW are as good as DL's best, and frankly I think the worst ones are better than the worst we ran into on DL. (Very few not-good in both cases, I must say!)

One caveat to the original poster: NW domestic has no IFE. This is a feature, not a bug, for us. Others miss the electronic babysitter. Ah, I see someone else mentioned this. We don't draw the same conclusion from the fact, however. Tastes vary.

Hmmm, it’s been a while, aren't DL seats blue and the walls sort of beige? Not all that different from NW, as far as I can recall, except the planes were often dirty, and had windows so scratched and dirty you could hardly see out of them. At least NW’s planes generally seem fairly clean. I rarely have to spend time brushing off my seat before I sit on it, which was pretty nearly sop on DL. NW’s F seats also often have footrests, although I still carry my own because Lex likes the bulkhead, which, as mikey pointed out, usually has deep cut-outs and has more leg room than DL's bulkhead or CO's.