Hrm. In 3rd Edition, I'd guesstimate....
Fighter Level.........Average Damage Per Hit
1.........................15 (greatsword, power attack, 18 Str; adjusted for crits 10% of the time, the average damage is 15.15)
5.........................21 (same but +1 greatsword and PA for -2/+4, and weapon specialization, Str 21 from 4th-level increase and Gauntlets of Ogre Power; adjusted for crits 10% of the time, the average damage is 21.21)
10.......................36 (same as 5th-level but +1 holy greatsword and PA for -4/+8, and improved critical, and melee weapon mastery, Str 24 from 8th-level and replacing Gauntlets with Belt of Giant Strength +4; adjusted for crits 20% of the time, the average damage is 36.58)
15.......................58 (same as 10th-level but +4 holy vicious greatsword and PA for -8/+16, greater weapon specialization, Str 27 from 12th-level and replacing Belt with Belt of Giant Strength +6; adjusted for crits 20% of the time, the average damage is 58.88)
20.......................69.5 (same as 15th-level but +5 holy vicious wounding ghost touch greatsword and PA for -12/+24, weapon supremacy, Str 34 from 16th and 20th-level and using a Manual of Gainful Exercise +5, assuming the average foe has 20 HD and thus loses 1 hp/HD from every second hit with the wounding weapon; adjusted for crits 20% of the time, the average damage is 70.62)