In answering this question, I'll restrict attention to the most often chosen first colors, blue and black (and blue-black).  I'll consider three criteria:

Reliability (R): Some inks will work well in most any pen you put them in. Others won't. Since I want you to have the best possible first fountain pen experience, this is a key criteria. The rating scale is Excellent, Good, Fair, Poor.

Flow (F): Some inks are free flowing and will give a wetter line for relatively dry pens. Others are less free flowing. Some pens that already have generous ink output do better with less free-flowing inks. The rating scale is: Free, Medium, Not Free.

Saturation (S):  The more saturated the ink, the darker and less watery the color. The rating scale is: Dark, Medium, Light.

Waterman Florida Blue. R: Excellent; F: Free; S: Medium;

If you only own one bottle of ink, this should be the one. Works well in pretty much every pen, looks good, lays down a wet line. The only drawback is that Waterman ink has a funny smell that some people dislike.

Waterman Blue-Black. R: Excellent; F: Free; S: Medium

Another nice Waterman color. Takes you back to bygone days of sleeveguards and eyeshades. A good choice.

Waterman Black: R: Excellent; F: Free; S: Light

This is kind of a washed out black. It's reliable enough, but not a nice color.

Aurora Blue. R: Good; F: Medium; S: Dark

This is a darker and deeper blue than Florida blue. It's very attractive. Will not perform all that well in very dry pens.

Pelikan Brilliant Black. R: Good; F: Medium; S: Dark

This is a nice dark black in relatively wet pens. In drier pens, it can be more grayish, which is not nearly as nice. Overall though it's a better black than the Waterman.

Sheaffer Skrip Blue. R: Excellent; F: Free; S: Light

A perfectly reliable ink, but the color is a bit washed out.

Levenger Cobalt Blue. R: Poor; F: Not free; S: Dark

This ink is very seductive. When it's working well in a pen, it's a gorgeous deep shade of blue. There have, however, been many reports of pens having problems with this ink. Stay away.

Montblanc ANY COLOR. R: Poor; F: Not free; S: Light

Despite its pedigree, this ink has nothing going for it. Avoid.

There are a number of other places on the Internet that review many more inks. Here are some highlights:

Fountain Pen Ink Sampler

Glenn's Pen Page

Pentrace Inkspot

Got an oPENion? Write me at sonnetude@go.com