i just read MASSIVE sections of the "principles of brain evolution" textbook. 75% of it in several hours
DAMN it was interesting. and it confirmed MANY of my suspicions. that larger animals (within the same order) ARE generally smarter than smaller animals in the same order (making brain to body mass ratios somewhat suspect metrics for intelligence). also, cartilaginous fishes are actually the most highly encephalized vertebrates ALL the way up to mammals (not even the monitor lizards have similar brain to body mass ratios) - and their brain to body mass ratios even rival that of birds. book does a good job of mentioning manta rays - they're one of the hugest mysteries in brain evolution - they're basically as unusual as octopuses, but no one knows that they're so damn brainy.
what's INTERESTING: amphibians truly are dumb (even dumber than fish). lungfish are pretty small-brained for fish, but amphibians are EVEN smaller-brained. also surprising is that salamanders and newts are even SMALLER-brained than frogs.
snakes are also remarkably small-brained, but textbook is careful to say that it's probably the result of gigantism, and is in no way a reflection of lower intelligence. after all, snakes don't even have limbs, so they could conceivably use more of their (comparatively smaller) brains for higher-order functions (and this is true for dolphins and whales too).
also, hammerhead sharks are remarkably big-brained. and great-white sharks are remarkably small-brained (they fall below the allometric scaling curve for cartilaginous fishes)
the most highly encephalized fish are elephantfish but most of it is for their special electric organs. But now I have an idea of where to look for intelligent fish behavior: triggerfishes. interesting that eels and moray eels have unusually small brains, especially given that moray eels are the only fish (so far) to be observed participating in interspecies cooperative foraging. again though, wormlike animals in general have unusually small brains, which may have NO reflection on their intelligence simply because they don't need special brain regions for their limbs (and grow unusually long anyways)
also, ostriches have the largest birdbrains. even though they're among the dumbest birds. pelicans have bigger birdbrains than macaws. but macaws do fall above the allometric curve for most birds (interesting that owls, parrots, and songbirds pretty much have the same allometric curve slope, but with a similar y-intercept that is consistently higher than that found in birds).
and the diagrams were SO interesting and i should post them sometime
also, google books for a recent 2011 book shows that even though Portia spiders SEEM to be remarkably smart (despite tiny brains), it just means that they evolved towards following efficient computer science-y algorithms. algorithms selected for novelty. just like ants. in other words, the insects with complex behavior aren't really sentient in any way. they were just selected to follow the most efficient computer science-y algorithms