Templars? Roman Legions? Spartans? Iraqis? Who were the best?
And I know it's hard to compare Roman legions to modern-day soldiers, so maybe you could break it down for each time period. So name the best from the ancient times, medieval period, late medieval period, that Napoleon-ish period (can't remember the name) 20th century to present.
You're right about the difficulty of comparing ancient warriors like the Romans to modern soldiers. In ancient times, nearly all the combat was hand to hand. Yeah, they had archers and catapults, even flaming tar pots (the ancient equivalents of machine guns, artillery, and napalm), but most of the fighting was up close and personal. So in my opinion, the ancient warriors were all Billy Bad-Ass compared to our guys today. Hell, when I was in Vietnam, we had snipers shooting people in villages where they couldn't even see the freakin' villiage without a scope, much less the people they were targeting! And don't even get me started on the guys carpet-bombing from 50,000 feet. That being said, the Spartans were probably the baddest of the bad in ancient times. In most societies the warriors were a special cast, but in Sparta, they were all warriors (or at least trained to be). The whole state was one single military industrial complex. But I'd have to give honorable mention to the ancient Germans, as well. A Germanic chieftan named Herminius (or Arminius) as the Romans called him, slaughtered 3 Roman Legions in the Teutoburg Forrest in the year 9 AD. The Roman Empire was permanently crippled. The boundary between the Roman Empire and the Germans was set at the Rhine River forever afterwards. It should be noted that "Herman the German" as he's usually called, fought Rome when it was at the hight of its military power, under the Emperor Augustus (Julius Caesar's nephew Octavian), and not when its power was pretty well spent, like the Goths, Vandals, et al.
Picking someone from Medieval times is difficult. Most of them sucked big time. But if I had to choose one, it would the the Muslim Chieftan Saladin, who was far more of a gentleman knight than Richard the Lionhearted ever was. Richard once slaughtered some of Saladin's men AFTER they'd surrendered. Later, when Richard found himself a prisoner of Saladin, he expected the worst, but it never happened. Honor was more than just a word to Saladin.
Modern times, I'd have to pick United States Marines. I may be a bit biased, having served in that organization myself, but I'm trying to be objective. What other modern fighting force has had a success rate anywhere near U.S. Marines? From the time Lt. Presley O'Bannon marched his men through 800 miles of desert to attack the city of Tripoli from behind (the guns were pointed toward the sea, since the desert was considered impenetrable before O'Bannan's exploit) to the Battle of Belleau Wood in World War 1 (where Marines fought so fiercely the German commander called them "teufel hunden", or devil dogs -- a name Marines have worn with pride ever since!) to the South Pacific Campaign of World War 2 (places like Gaudalcanal, Pelielu, Tarawa, New Britain, and others too numerous to mention) to Korea, Vietnam, and now Afghanistan and Iraq. Marines have done it all, on land, sea, and in the air. Nobody else can claim to have more than two of those missions covered. Marines do it all!