What inks work best for flexers? Typically, they require a flow that is neither too slow (causes skipping during the flexing requirements), too fast (it won't snap back to a fine line after flexing quickly enough) and CAN"T feather or spread. I just tried Legal Lapis and it was too flowing.

My favorites:
Apache Sunset
Pelikan blue-black
Lamy blue-black
Sheaffer NOS Lavender (discontinued but J. Herbin Violet Pensee is nearly identical and works just as well)

As you can see, I think "shading" inks are best in flexible nibs. But that's just me. Based on that, Just about any of the original J. Herbin inks will likely do well.
Maybe MB inks since they're a bit stingy.

Actually, I loved Levenger Cobalt in my Falcon. But it's blotter-needy on some papers.

So far, FPH Old Mahattan Black & FPH Ellis Island perform well in my flex pens. However, I don't recommend Tulipe Noire since the flow isn't as generous which makes "outlines" somewhat more common.

The Waterman inks work great in my flex pens. These are generally easy-flowing and fantastically lubricating inks, which is great for the demands of a flex nib. They don't tend to feather, either. They respond well to various levels of flex and provide great shading.

Of course, inks like Noodler's Marine Green, Herbin Violette Pensee, and Pelikan Brown are great as well. You might have to try a few to see what works best in your particular pen.

I've never had a problem with slow flow in a vintage pen with a flex nib regardless of ink brand - they seemed to know back then how to get the flow right for the nib - nor have I had a problem with "snap back" (isn't that more a function of the nib than the ink?). That said, I tend to use what I think of as rather watery inks - e.g. Diamine and even Waterman - in vintage pens with flex nibs because they're the only pens wet enough for such inks to leave a line that's not insipid and, in such pens, you get marvelous shading (some inks more than others, of course). I've not had feathering problems with any Waterman ink (or home-made mixes), but have with some Diamines (Blue-Black being the worst). I wouldn't use a slow drying ink (unless it's diluted) in such a pen because they're such wet writers.

As for modern pens with flex, everything seems to work well enough in Namiki Falcons, whereas the big Pilot Falcon doesn't quite work with any ink (as I've mentioned before, I'm looking forward to getting mine back from Richard Binder to see whether his "fix" works).

Simon

Waterman (Blue, Havana Brown, and Blue-Black)
Diamine (most I've tried, except for Blue-Black)
Herbin (Eclat de Saphir, Bleu Nuit, Cafe des Iles, Terre de Feu, Orange Indien, Larmes de Cassis)
Sailor (Red-Brown, Blue-Black, Brown)
Platinum Blue-Black

To reply in my own thread, I think thinner, somepwhat drier, inks work quite well. I just switched from the poorly behaved Legal Lapis to MB Brit Racing Green. This ink works superbly in my Falcon (although delivering less of a gliding feeling). Similarly, I use Pilot blue-black in my Custom 743 with FA nib. Both these inks give the added benefits of water-resistance and shading.

Lately I've started watering some Noodler's inks, about 2:1 (water:ink) in a vial, for flex. Suddenly it's okay to use with flex (which tends to run wet on the downstrokes, and then feathers or dries slowly, or never dries). Watered-down will give nice shading, and won't dry so slow. Feathering will depend on the paper you use. The nib will be the opposite of smooth, however.

PR Grey Flannel is fairly dry and thin, and somebody recently posted a review done with a flex nib that was stunning.

ETA: I think that may have been a non-flex italic, but it's still a thin and dry ink.

I particularly like any of the R & K inks as they all provide a good amount of sharding - very partial to Alt Bordeaux and Gold Grun (the correct name is escaping me right now but the yellow/green one!). Also as other FPNers have already attested, J. Herbin is great as well.