It should be the genetically-engineered fighting machines that are the Jem Hadar.
The Hirogen may be second, if we're talking about the BIG ones, and given they've survived for millennia and stretched across two quadrants.
After that, I think it's a toss up between the Klingons (bodies can take more abuse, and their empire's expanded further in less time) and Vulcans (stronger, smarter, and future looks brighter).
The Hirogen may be second, if we're talking about the BIG ones, and given they've survived for millennia and stretched across two quadrants.
After that, I think it's a toss up between the Klingons (bodies can take more abuse, and their empire's expanded further in less time) and Vulcans (stronger, smarter, and future looks brighter).