Wow! Didnt know you guys knew that much about Spartans...
I just have to be different...for ancient fighters, Id have to go with the Knights of Malta. They had a smaller force than the spartans, and kicked quality butt in their fortress for years.
For modern Times, Id have to say the Republic of Korea (ROK) Marines Forces stationed in Viet Nam from 1966-72.
Of course, i have noticed that no one has even acknowledged the Swiss Mountain Troops for toughness. They actually can play just as mean and hard as some of the best special forces groups in the world.