I really recommend a Noodler’s Ahab. They’re not the easiest to get hold of in the UK (but not impossible) but they’re cheap, you can completely disassemble them for cleaning, and you can flex them:

Which means you get some great line variation.

It also means you lay down a lot of ink, which will affect the colour of the strokes and in some inks (like this “Deep Dark Purple” from Cult Pens) you get sheen.

Not the best pictures, but you can see that the sheen is a really nice shade of dark green on top of the purple (don’t tell the Drazi).

The only issue with flexing is it takes so much ink at once that sometimes the feed can’t keep up. But also some Ahabs just don’t like some inks — it may have something to do with the ebonite feed, rather than a plastic one. Here it runs dry on heavy flexing.

Of course I was really pushing the flex on that one — normally when I write the result is more like the middle line. It still lays ink pretty wet, but that can work with e.g. laid writing papers. Gives a lot of character to the writing, even if you don’t know how to do copperplate.