Perhaps it's because there is so much variation, even within one brand, that the question is difficult to answer. I have favorite inks from many different brands, so not much brand loyalty. You really need to add Diamine to your poll.
Some of my personal favorites at this moment are:
Diamine Ancient Copper
" " Merlot
" " Meadow Green
Noodler's Henry Hudson Blue (Fountain Pen Hospital exclusive)
" " Purple Heart (Goulet Pens exclusive)
" " Squetegue
" " Upper Ganges Blue
Pharmacist's Turkish Night
" " Darkening Absinthe (Both hand made iron gall inks from Belgium)
I guess I have more Noodler's ink than any other brand, but I would hate to be limited to one brand. One thing that Noodler's has above anyone else is a good range of fully waterproof inks, that chemically bond to cellulose and won't wash off the page. This is good for work environments that include water spills or puddles of condensation from pitchers of ice water. Also good for writing checks or addresses on envelopes or packages that might be exposed to rain.
Ulises Victoria --- You really need to go over to the Fountain Pen Network Forums for this sort of information! There are two ink forums over there, one for ink chatter, and one just for reviews. I think you will like it.
Inky Thoughts - The Fountain Pen Network
And...
Ink Reviews - The Fountain Pen Network
Dan