Better drop Totenkopf (3rd) off the list. Most importantly, they had loads of sadistic concentration camp guards on their payroll and routinely rotated personnel between the camps and the division. Militarily, they were a sorry bunch in France in 1940 and still in Russia in 1941, more known for their brutality against POWs than combat prowess. Then they improved, being nearly annihilated at the siege of Kholm in the winter of 41/42, but in the end withstanding all Soviet assaults. After getting their ranks filled with a new stream of psychopathic killers, they then bled white against the Soviet defenses at Kursk in 1943. And in-between, managed to slaughter countless Russian civilians as "partisans". If I remember correctly, the division's final mission was the unsuccessful recapture of Budapest with the 6th SS Panzer army in early 1945 after which they eventually surrendered to the Soviets and were mostly "put to the sword". And every one of them would have deserved this fate.
I'd be carefull taking "war crimes" into the comparision. Expecially when Russian units are involved. Russian troops comitted horrible crimes on the eastern front. Totenkoph certianly started off slow and did at first have a rotation with concentration camps. But, by 1943 it was as good a combat formation as any in the Waffen-SS. Any defeats it suffered late in the war were not becuase its performance.
The Huron and Iroquois warriors during the seven years war are in a league of their own. They were efficient guerilla fighters, brutal, and extremely effective.
That is putting it mildly. I was doing some research for a paper when I was in University and came across letters home from soldiers in that war. They had been transferred to microfiche so we could have access to them. Every one that mentioned the natives did so in such a way that you couldn't help but feel that the author of the letter was absolutely terrified by the natives and their unique way of fighting.
They aren't really organized into a regiment or anything but they still deserve consideration in this context. Great warriors from a bygone age.
I personally wouldn't include units that proved their "toughness" or better, acquired a reputation of being tough, in only a single combat engagement, as some here did.
Also I think it's quite difficult to determine that if you have a time span of 300 years. Some units fought barely a year, others fought 5, some units won a single decisive battle, others showed their worth time and again in many less important ones, some units just made best use of their technological advantage, others simply had the better soldiers.
For me personally it would be the 2nd and 5th Waffen SS Panzer divisions. Given the odds against they were fighting in the last years and the lack of almost everything, they sure put up a tough fight. The only atrocities I could find were commited by elements of the 2nd SS Panzer. But taken that most people think that the whole waffen SS were murders, that's probably not important.
About Totenkopf they surendered to Americans, but they turned them over to Soviets, becouse they were ''already full of prisoners''.
About toughnes. Third batallion of Prešerns brigade was (thanks of traitor) suprised in ski hotel, resting after long winter march. German attack on hotel failed, so they set it on fire with mortars. Several groups of partisans tried to break out but were cut down by machineguns. Germans were pounding hotel with everything they had and offered partisans chance to surrender but they refused and continued to fight from the basment of the burning building. On two ocasions partisans managed to germans into coming close to basement (they stoped firing and germans thought they are dead). At the nightfall, those few that still could walk tried to breakout and those that couldn't shot themseves. Germans then finished those wounded that were left from breakout attempts.
They did but ,all jocking aside ,and considering there were a lot of ex Waffen SS in their Ranks,they fought a losing battle against overwhelming odds,but they fought hand to hand,and in some of the bloodiest fighting of the Indo China war