A poll generated from: http://www.trekbbs.com/showthread.php?t=215668 Who is the toughest Star Trek species? The Metron, the Trelane, and the Q are left out for obvious reasons.
I voted Hirogen...in complete disregard for the rules of this poll. You see, the word 'toughest' to me doesn't imply 'who would win a war', it suggests who would possess the physical and mental toughness to prevail. Too many of the races mentioned are only formidable due to their tricks. The Founders are shape shifters [I see no real evidence of 'toughness' from them], the Doud...we know very little about, what we do know is they are reclusive explorers. [not 'tough'] The Borg are bad-ass due to their technology and fact that most of them are mindless drones. The Prophets are god-like and frequently stand-offish. So I vote the Hirogen. Who are frequently up for a fight, will relish it and have the physical and metal stature to continue it.
It kinda does, considering that they're the creators and custodians of this universe. I mean, it should be noted that every species or race on this list were fundamentally affected by humans in some form or shape, when in reality humanity would have been wiped out several times over, from invasion to plague to a mere finger snap. In the grand scheme of things, species' power levels are inconsistent across the board, and are written only to serve the plot and the goals of the writers (8472, for example).
My initial reaction would have been the Douwd, since the one depicted seemed within the realm of Q-level power, but didn't seen omniscient like Q seem. The Prophets also have Q-level ability, but seem to be able to manipulate time and space at least in a limited area (including sealing themselves off from the Alpha Quadrant and residing in the Worm hole), and there are more of them than the single Douwd that was encountered, so I would have to give it to them over the Douwd.
In terms of tangible species, I'd say Yarnek and the Excalbians. Tough like the Horta, but also capable of mental powers like some of the above species. The only thing else I can imagine that comes close (non-Trek) is the Cheela http://www.daviddarling.info/encyclopedia/N/neutronstarlife.html
One single Dowd wiped out an entire civilization all by himself, evidently in little or no time, without the use of any technology whatsoever.
He was devastated by the death of a single woman. Incredible abilities yes, but not so tough. Guess it depends on what you mean by the word "tough."
Going by that list: Tin Man - The last of its kind, wandering the galaxy with Tam. Not much of a force to be reckoned with though able to defend itself when needed. The Douwd - Were interested in peace above all else, grief and anger caused one individual to take action. I doubt they'd be interested in battling anyone though. The Kelvan - Able to travel across galaxies, so they would probably be a force to be reckoned with--if they wanted to fight that is. The Prophets - Totally uninterested in affairs outside of their realm--only took action after being pestered by The Sisko. The Pah-Wraiths - Only interested in revenge against the Prophets. Species 8472 - They seemed trapped back in fluidic space. It they weren't they would've attacked by now. The Borg - VOY destroyed them as being a threat, since one solitary ship could defeat them on multiple occassions. The Hirogen - Nomads, no collective force except for the occassional hunt. Not interested in battle just hunting. Remans - Their only focus would be against the Romulans, not anyone else. Caretakers - No clue where they are. Again able to cross galaxies and very advanced, but not much interest in conflict (from what we saw). Founders - Created the Dominion. Nuff said really. Humans - Kinda feeble without the rest of the Federation.