300 years so for those not too great at the old maths, or math as you Americans say for some peculiar reason, then thats from 1703-2003
Thats a lot of military units to cover. Anyway its kind of pointless as well because technology changes, politics, social stuff etc etc but just to amuse myself then i would say that some of the British units in the Napoleonic Wars were pretty tough. Some of the Irish units, Cannuaght Rangers were scary people as well.
Britsh infantry at Albuera
as Marshall sout said of them "The Day was Mine and they did not know it and would not run"
royal marines and Paras in the falklands "well the airlift is now at the bottom of the sea so your walking crack on "
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.
German Fallschirmjagers at Monte Cassino "The Green Devils" as they were called by the allies. Merills Maruaders, the Chindits.
Earlier, Rogers Rangers, Francis Marrion and his boys, Union Sharpshooter Regiments.
Fraser's Highlanders. At the battle of Quebec in Sept. 1759 they fired their first volley, and rather than reload and shoot again, they drew their broadswords and charged the French, causing a panic even among the native allies, which were at the time quite fearsome to both sides. Their charge was quite startling.
Other than that, I would say anyone who landed in Normandy definitely sticks out.
Marines at Iwo Jima. Defenders of the Alamo. Rangers at Omaha Beach and Point Du Hoc. Canadians at Juno Beach. 101st at Bastogne. Marines at Wake Island. Task Force Ranger in Somalia. Russians at Stalingrad. (Germans put up a hell of a fight too.) And last but not least, the 1st Calvary at the Ia Drang Valley.