The Long-Tail Cast-on it is
I knew I wouldn't be happy just guessing which to use, so I swatched each of the cast-on methods I was thinking of.
From top to bottom, we have the tubular cast-on, the cable cast-on, and the long-tail cast-on.
The stretchiness of the cast-on edge also goes from top to bottom, and although the long-tail cast-on is not nearly as stretchy as the others, it's still rather stretchy.
For my swatches, the tubular cast-on just looks so loose and messy. I don't like it at all. Knitting off the cable cast-on gave me looser ribbing than knitting off the long-tail cast-on. Admittedly, I was having trouble with tension while I knitted it. For some reason, despite this being my 3rd swatch of the ribbing, I couldn't keep any tension on my yarn while I knit, and I have no idea why.
I had to do the long-tail method twice. The first time, I did the first row after the cast-on in the 2x2 rib, but started with the wrong side, since the long-tail method essentially gives you a first knitted row. I didn't like the ribbing coming right to the edge of the fabric, so I re-did it with that first row just being a purl row, and then starting on the 2x2 rib after that. The edge looks much more professional.
In the end, the swatch I did with the long-tail cast-on just plain looks the best, so that's what I'll use for my Ribby Cardi.
And although my swatches might turn out different than anyone else's, I hope this little experiment might be helpful to others, as well.