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In a word cetaceans. Some have bigger brains than humans.
The encephalization quotient that Galaxy cites isn't a direct measure of brain size, it's a ratio of actual brain mass to the value that a mammal is expected to need in order to manage things like breathing and coordinating body movements. Cetaceans and particularly dolphins do have some of the beefiest brains among mammals in terms of EQ, but humans still beat them by a wide margin.
Ants then. They are 6% brain, which easily trumps humans in terms of EQ.
The idea that there's no trend towards bigger brains seems stupid. Almost as stupid as the idea that we should expect of see such a trend in all organisms.
All modern organisms evolved from bacteria-like critters, with little nervous tissue. Of course there's a trend towards bigger brains. The only issue is why - and that seems pretty obvious too: brains pay.
A random walk away from no brain by a large number of lineages would also give you a constantly increasing upper bound on brain size.
That was S. J. Gould's model - in Life's Grandeur. Assuming that a trait like brain size behaves as though it is a neutral trait seems pretty loopy, though. Now we have enormous data centres to explain, the "random drift" model is surely no longer worth entertaining.
That doesn't fly either, I'm afraid. EQ is not a straight brain-to-body-mass ratio; the expected brain mass in the formula scales sublinearly against body mass, giving smaller animals a tendency to have proportionally larger brains. This is motivated by the fact that some nervous functions (diaphragm contraction, for example) are relatively constant in complexity but still have to be handled by small animals' brains, so the smaller an animal is the more nervous overhead it has to deal with.
The formula this works from was only designed to apply to mammals; different body plans have different neural requirements, and arthropod nervous systems are very different from mammalian. If we ignore that and apply the mammalian formula Ew(brain) = 0.12w(body)^0.66 to something ant-sized, it gives us an expected brain mass of about three milligrams for an ant weighing four milligrams (obviously absurd, but we knew the formula was bogus) and an encephalization quotient of about 0.08 if your 6% value is accurate. Human EQ is about 7.5.