The question as posed is impossible to answer. First of all because of the massive divide caused by the arrival of fire-arms. It completely changed the ballgame.
Second an unevenness in sources for information for both era's and theatres.
So I can only partially answer your question:
For ancient times, my vote whote go to the Germanic tribes, and more particular those living along the lower Rhine. And these are my arguments in their support:
From the time of Caesar on these were seen as the finest warriors around. Since long they formed the guard units of many rulers. In battle they were as tough as they come, especially when armed with Roman technology (mail and helmets) and for a while Roman training.
Also, the Germanic (and Danubian) legions and auxiliaries were throughout the Imperial era and the Late Roman Empire considered the crack troops of the Roman army, because they had to face the Germanic tribes. In fact many of them were of Germanic origin themselves, and in the fourth century the corsp of the Roman army in the west (both in numbers and quality units) was German, from th Lower Rhine (in this case Franks). Just a few examples of their prowess and praise throughout the centuries:
In 52 BC it was a charge of Caesar's Germanic Horse Guard that saved his army against the Gauls. Their 400 men made the difference. It is telling that this horse guard went with him wherever he went afterwards.
Well known is the massacre of three Roman legions under Varus by the Cheruscans in 9 AD. Telling is as well that the Romans still used Germans in the auxiliaries and the guard.
In 39 AD Caligula used Germanic auxilaries (Batavi) to squash a revolt of the Roman garrison at Mainz.
In 68 AD some Germanic tribes revolted (the Batavian Revolt). The defeated several Roman units, and it took a full scale war for the Romans to achieve victory. Remarcable though is that
this is the only known case in Roman history where the tribes were not severley punished (all but exterminated). In fact, the situation was restored to the situation before the war. The Romans in fact acknowledged many of the grievances of the tribes that lead to the revolt.
Even more siginificant is that the Romans even after the revolt allowed these Germanic auxiliaries to be commanded by officers of their own tribe!!!. Auxiliaries being commanded by native officers was a rarity in itself
In 83 or 84 AD at Mons Graupius the Batavian (Germanic) auxiliaries utterly destroyed the Caledonian forces (allegedly the Roman legions proper never even needed to get into the action). In fact, it seems that most of Agricola's army (12.000 out of 20.000) consisted of Germanic auxiliaries (cavalry and infantry).
So within 15 years the same tribes that revolted and burned everything Roman to the ground between the Rhine and northern France were part again of the crack troops of Roman army!
Under Trajan the Germanic Horse Guard was reinstituted (perhaps even under Domitian). Trajan refounded the Batavian capital (Ulpio Noviamuagus (Nijmegen, Netherlands).
The emperor Hadrian composed poems in honor of their prowess (swimming the Danube in full battle gear) and probably for a number of their epitaphs.
Under his reign the demonstration of his Batavian Horse Guards swimming the Danube in full battle gear was enough to cower the tribes on the other side.
In 312, the core of Constantine's army consisted of Germanic troops. In fact, on his arch a number of the panels depict Germanic soldiers.
In 351 at Mursa Major the army of the usurper Magnentius, with at its core troops of Germanic origin, faced the army (battle hardened in the east) of the emperor Constantius which was almost twice as big. In the end they did lose, but it cost Constantius half his army in dead (30.000).
In 357 at Strasbourg Julian defeated with a much smaller army the Germanic traibes who had invaded (and completely overrun) much of Gaul. His army largely consisted of newly recruited men... all of Germanic origin.
According to Ammianus Marcellinus, a Roman army officer (veteran of most campaigns of the Emperor Julian among others) and historian of the third century and possible eye-witness to the event, a demonstration of less 500 Germanic horse guards swimming the Tigris fully armored was enough to compell the Persian Shah to offer peace. The same Shah who shortly before had inflicted a crushing defeat on the Romans.
In his narrative about the events that would climax in the Battle of Adrianople (378) he makes the following statement (paraphrasing): "He (Valens) sent some units from Armenia west to face the Goths. These troops had distinguished themselves in the wars agains the Persians and were some of the best in the east. However, compared to western troops they were completely inferior,
as they had never faced the onslaught of the Germanic tribes." (Italization my own addition).
In 394, at the two day battle of Ad Frigidus, the Theodosius squared off against the troops of the usurper Eugenius. The army of Theodosian was decidely larger. The army of Eugenius again had Western troops and thus Germans at its core. The Gothic auxiliaries of Theodosius were massacred on the first day. In fact, Theodosius was (probably) saved
by the fact that some of the Western troops defected to him.
All throughout the 4th century Roman authors comment on the prowess in battle of the Germanic tribes (veriously termed Germanic/Gauls), and almost put them in a class of their own. Indeed, many of the most prestigious, elite and oldest units are of Germanic origin, have Germanic names and sometimes also wore German emblems.
So I feel that for the Romans the Germans (and the troops that faced them) quickly became the measuring stick just as Alexander, Hannibal and Napoleon have become the measuring stick for battlefield commanders
I hope I have made a good plea...
Harm