No need to iterate or loop. You can spot the LastRow (LR) and LastColumn (LC), store them in a variable, the insert the formulas all at once using the R1C1 syntax:
Code:
Option Explicit
Sub AddFormulas()
Dim LR As Long, LC As Long
LR = Cells(Rows.Count, 1).End(xlUp).Row
LC = Cells(1, Columns.Count).End(xlToLeft).Column
With Range(Cells(LR + 1, 1), Cells(LR + 1, LC))
.FormulaR1C1 = "=MAX(R2C:R[-1]C)"
End With
End Sub