Mine is Persicaria amplexicaulis, a clump of which I was gifted over thirty years ago and from which every single plant I have since used is descended. It’s extremely tough, the only plant to grow in the dust below an elderly Cordyline australis we inherited and integrated into the design of our original London garden (below in 2012).
It copes as much with the damp of the Lake District, such in my parents’ garden in the Lake District (at the top of this page in 2015), as the heat of a London garden, such as my sister’s in Hackney (below in 1992).
It can be used as part of a no-dig approach by planting densely in the first year to clear a site of weeds as effectively as a layer of mulch, such as a front garden I am working on with a neighbour (below in 2019).
It’s irresistible to pollinators, particularly smaller ones such as hoverflies and scissor bees, so ideal for the guerrilla gardens in the tree pits (below in 2019)
and under the trees in our present London garden (below in 2017).
It is also ideal for bouquets (one from 2019, below).
BUT, it’s hard work.
If you turn your back for a year, it will overtake anything and anywhere. I dig up every plant every year, trim its roots and replant it so that it will grow only two or three stems. But I would not be without it.