Hunt 22B extra fine. This nib is on the stiff side (but not too stiff) and is similar to the Hunt 56 in feel (I'll be reviewing this nib in an upcoming post).
The Hunt 22B makes nice hairlines and thick strokes and it's not too difficult to write with.
I like it.
Speedball recommends the B style of holder but I used the Brause wooden holder. With the B style Speedball holder, the nib comes too far up the holder and it is hard to get ink in the vent area without getting the bottom of your pen holder dirty with ink (which can led to ink on fingers, smudges on paper, ink marks on your face, oh the tragedy....) so I prefer to use the Brause holder with this nib.