Use Condtional Formatting. Highlight B3:B6, and, from the menu, choose Format>Conditional Formatting. In the Conditional Formatting dialog box, for condition 1, choose:
Cell Value Is....equal to....=MAX($B$3:$B$6)
Click the Format button and set the font to Bold. O.K., O.K.
EDIT RE: Using VBA on opening: Is the range of the numbers you want to conditionally format always going to be the same? If so, just make sure your opening macro doesn't clear the conditional formatting and the maximum number of the new data entry will still show in bold.
will bold it if there are values and ignore it if no values.
If you really need code then something like
Dim rng As Range
Dim cel As Range
Set rng = ActiveWorkbook.Sheets("sheet1").Range("B3:B6")
For Each cel In rng
If cel.Value = Application.WorksheetFunction.Max(rng) Then
cel.Font.Bold = True
Else: cel.Font.Bold = False
Though thinking about it I guess it doesn't matter about catering for them all equalling the max (ie when they are all blank), so Oaktree's first formula with absolute reference for $B$3:$B$6 should do you.