A good BW will spread to every place in your garden where you are too lazy to plant anything, and will out-compete the weeds that you don't like.
My favorite BW is yarrow. Yarrow has a noble history: yarrow stalks were traditionally used to cast the I Ching. Also, I think yarrow looks pretty good. A 100-square-foot section of yarrow is more socially acceptable than the equivalent area of dandelions.
Another BW that works for me is Italian Parsely. It doubles as salad material.
Some plants are almost BW's but don't quite work. Coreopsis spreads well but looks strange growing in odd random places where you didn't intend it. Chamomile is a great plant but lacks toughness and invasiveness. Bamboo is tempting, but it is overkill, like the nuclear weapon of BW's. I keep it in