Why were the roman legions considered the best infantry in the world?

Highly trained and disciplined.  
The soldiers saw themselves as citizens of Rome.  
If a soldier fought for so long and lived then he could get land.  

Troops had roads they marched on, so they can move quickly.  
Troops were well fed and taken care of, life span lasted longer then living in the capital at one time.  

Troops could advance in their ranks by their merit.  
Troops can split the spoils of war.  
Worked together as a unit, a team to meet a goal.  
Troops were well supplied, and armed.  
Troops were given special honors from Rome.