Coffee contains a bunch of healthy compounds. Quercetin for one. A bunch of other flavonoids.

Coffee, the brew, is not a significant source of Quercetin. The grounds, the part you throw away, may be. But capers are like ~200mg per 100g, and outpaces all the other common sources of it, so if you were really big on Quercetin, you'd be looking at anything but coffee.

I do. I use Quercetin as my source of Quercetin.