Possibly one of the most fittest and highly disciplined of the Ancient World, was the Roman Legions.
The ordinary Legionaire in the early empire period, 1st to 2nd Century AD spent around 3 to 4 months doing basic training. They also did regular forced marches and field exercise training several times a month, and were probably much fitter than most average soldiers of today.
They also learnt various other skills such as fort building, road building, weapon making, armourers etc along with their weapons skills with short stabbing sword (gladius), spear (pilum), javelin and various battle tactics practiced in the ancient world. Military life was usually short and brutal, and most Legionaires who passed basic training signed up to 16 or 22 year hitches. Those who survived their service, often were a trained reserve that could be called upon in an emergency.
Their officer corps were highly educated, and the backbone of the Legion was the Centurionate. Made of from the ranks by promotion for outstanding service, or often entrants from outside who 'bought' their way into the Centurinate. All very interesting from an historical point of view for anyone interested in the military history of the ancient world.
I reckon BEF 1918 would be pretty damn fine too, at the end of a BIG learning curve following the near-annihilation of the first BEF early on. French Army during 1914-18 and Soviet Army 1941-45 too, for the ability to just keep on going in the face of mahoosive losses.
I don't know how true this is, but I heard that the Viet Minh dismantled and carried their artillery pieces up the hills overlooking Dien Bien Phu and obliterated the French Para's down below. The French apparently thought that nobody could get guns up there. It was in the 1950s I think.
Daft exercise, IMO, but fun nevertheless... so here goes:
Impossible to call - time/ place/ historical- cultural- technological contexts!!!
In terms of all round capability/ combat effectiveness at various times I'd rate -
1. Ancient World - the Roman Legions just after the Marian Reforms.
2. Medieval - the cavalry of the Arab Conquest armies; the Huscarls of C11th England, & the Norman "All Arms" formations of the same period. Also worthy of mention - the later medieval Anglo-Welsh archers.
3. Early Modern - the Ottoman Sipahis & Jannissery Corps (before they lost the plot & became utterly decadent!).
The Imperial Spanish "Tercios" were arguably, in their prime, the finest military formations in the world in their time.
Japanese Samurai, and - in some cases - the so called "Ninja".
4. Modern - Rifle Regts of Wellington's Peninsula Army; infantry of BEF, 1914; Italian "Arditi" (and their equivalents in German & Allied armies) of 1914-18.
WW2: Commandos (and their offshoots); German "Brandenburgers"; US Army's "Alamo Scouts" of the Pacific Theatre.
The Royal Navy of the 19th century.
Nelson's fleet at Trafalgar destroyed a superior force without the loss of a single ship, despite being under heavy bombardment for an hour before being able to return fire. No nation could compete with the tactical capability and overwhelming skill of Jolly Jack Tar.
Peninsular War, General Crauford's Light Division. At the Battle of Fuentes de Onoro, they withdrew some 3 miles across open ground surrounded by French Cavalry and lost few casualties. Their marching pace ensured that the French Infantry couldn't catch up even when repeatedly forming square.
For that matter, the Peninsular War veterans are arguably the finest army fielded by the United Kingdom.