I probably now have more fountain pen ink than I will ever actually manage to use in my remaining lifetime. And yet... there's always room for more, it seems.
I have a fairly wide selection of Diamine ink in my collection; probably more than any other brand. They're a great work-horse ink and they come in a range of colours at very reasonable prices. Although my (not exclusively Diamine) ink cupboard includes some real bobby-dazzler colours (like a bright pink and some sparkly versions), the ones I use for everyday writing tend to be a dark blue (like the blue-black, or the Oxford blue which is particularly nice), black, and "dried-blood red" (which is actually a lot better than it sounds!), a.k.a. oxblood to more normal people.
For those of us who like this kind of thing, there's some nice shading to be had on many of the colours (less so on the jet black, for obvious reasons). They've never have a 'hard start' in my pens, but that's not at the expense of being watery. Nero stocks a decent range of colours (see the full range here) - more than enough to cover most tastes.
If you're looking for a good present for the fountain pen lover in your life, then there are also two gift sets available: the Diamine Music Ink Collection and the Diamine Flowers Ink Collection. Each contains a selection of 10 inks, in delightful 30 ml glass cube bottles, beautifully presented in a gift-box.
I really like Diamine. I find they're a good ink at a sensible price point. There are good reasons my ink cupboard features so many bottles of Diamine.