Who gave the Romans the most difficulty on the battlefield?

Any and all of them caused the Romans problems, Carthage inflicted a number of serious defeats on the Romans though this was before the 'classic' Roman legions and military organisation that we more familiar with were formed,

Britons were always a constant problem with up to 25% of the entire Roman military strength being deployed on the Island for much of the period it was under Roman control, which is a huge percentage when you consider just how small the provence of Britain was within the Roman empire.

Dacia was a problem but was ultimately pretty quickly conquered and absorbed into the empire, ending up being a major source of recruits for the legions in the centuries to come

Germania wasn't a major problem, the Romans wisely kept on one side of the Rhine and left the other side to them, only on rare occasions did they cross the Rhine into barbarian territory, once lead to their defeat at Teutoberg, but other than that their expeditions tended to be successful punishment raids before crossing back in to Roman controlled Europe.

Judea and the Jews were another thorn in Roman sides destroying an entire legion in their great uprising, before the Romans assembled a huge force and destroyed Jerusalem in 81ad,

The Sycians were a serious problem to the Romans defeating them on several occasions and and even capturing and later executing an Emperor, (who legends claim was then stuffed and used as a footrest)

I think its fair to remember and account for distance between the main center of the roman empire and the regions where they battled many of their foes. Although the scots and parthians proved to be thorns in Romes backside, both of them were incredibly far from the "known world". I think the Gauls, germans and or the carthaginians may have given romans the most difficulty in battle. Remember hannibal spanked the romans on the open field time and time again. And then their is Pirus of Greece....too many to chose from.

Mithridates VI of Pontus, who faced 3 of the Republic's greatest generals; Sulla, Luculus, and Pompey. Not only was he militarily able, even conquering some provinces from Rome--but politically able as well, in organizing a large-scale revolt against Rome, including much of Greece and other parts of the eastern empire.http://en.wikipedia.org/wiki/Mithridates_VI_of_Pon...

There is a good case for saying it's the Scots. For no other county did the Romans need to build a wall across the access to an entire country to try and keep their lands and possessions safe.

I would say the Gauls. My next choice would be Hannibal himself. I think the Gauls mounted such an effective and constant Guerrilla war the Romans almost lost.

They never did defeat the Parthians (Iraq) although they tried a number of times