You need to be careful that you grow a sterile comfrey like Bocking 13 - it does not produce seeds and so does not spread itself everywhere; It is only propagated by cuttings. If you grow comfrey from seed, it will reseed itself and be everywhere. Find someone with some Bocking, and they can put a cutting in a pot and it will be away in no time.
Other thugs of the herbal world: anything in the mint family (incl. lemon balm), fennel, and sweet woodruff. I potted up some mint cuttings for a friend and rested in on another (tall) pot so I did not forget it to pass it on. In two weeks it had roots out of the pot and 6" into the other! I usually keep the thug pots on hard surface but forgot that time... I got sweet woodruff yonks ago from a lady who told me firmly to put it in a place where it can't escape or at least in a restriced area. Smells amazing but will be everywhere overnight if you let it. Makes nice herbal pillows.
I like Sweet Cicely, and old-fashioned herb that seems to thrive under my quice. Nice anise-like scent and I hope to use it with rhubarb next year (to reduce sugar requirment).
The usual suspects, parsley sage, thyme, rosemary (I like the prostrate kind - less of a giant shrub), and lavender are fab - great for insects when the flower, lovely and useful. Lots of varieties of each to have a go with