copy data from one column to column on another sheet starting with first open cell

This will loop through all the worksheets if that's what you want.

AdvancedFilter assumes the top cell (B1 in the case) in the filter range to be a header.

Sub PopulatePlayers()
    Dim ws As Worksheet
    For Each ws In Worksheets
        If ws.Name <> "Overall Leaderboard" Then
            If Not IsEmpty(ws.Range("B1")) Then
                With ws.Range("B1", ws.Range("B" & Rows.Count).End(xlUp))
                    .AdvancedFilter Action:=xlFilterInPlace, Unique:=True
                    .Offset(1).Copy Sheets("Overall Leaderboard").Range("B" & Rows.Count).End(xlUp).Offset(1)
                    If ws.FilterMode Then ws.ShowAllData
                End With
            End If
        End If
    Next ws
End Sub

This copies from just the activehseet.

Sub PopulatePlayers()
    With Range("B1", Range("B" & Rows.Count).End(xlUp))
        .AdvancedFilter Action:=xlFilterInPlace, Unique:=True
        .Offset(1).Copy Sheets("Overall Leaderboard").Range("B" & Rows.Count).End(xlUp).Offset(1)
        If ActiveSheet.FilterMode Then ActiveSheet.ShowAllData
    End With
    MsgBox "Unique Players copied", vbInformation, "Copy Complete"
End Sub