What species is the Borg Hugh?
(Slightly related: What species is the Borg Queen?)
Hugh, introduced before the concept of species numbers was first used, was never described/explained as to which species he came from, in either canon or extended material. Hugh is simply a Borg Drone.
Within the main canon (TV/Films) Hugh's species is never revealed.
That being said, we get a little more info about Hugh in the EU novel "Greater than the Sum". Hugh seems to be a Heinz, incubated by the Borg and artificially generated using the genetic material from multiple species;
Hugh had already told the new crew members how Geordi had shown him individuality and how Picard had sent him back to “infect” his fellow drones with that sense of self. He had told how his cube full of incubated drones, with no prior identity or social structure to fall back on
“Of course,” Beverly said, nodding. “Your people were incubated drones-essentially androgynous.”
“And the Zeros,” Grabowski said, “well, most of us had our reproductive machinery torn out of us and replaced with circuitry, our gametes harvested to be engineered into new incubated drones. Those few of us who retained the organs had our hormones and nervous systems messed with so much that we’re…no longer capable of sexual response.”