Return highest sales value Product per Client

StephenClarke 

You can add the following column to get the desired result. I found additional columns in your formula that was not in the sample data you provided, I hope you can adjust the following formula to suit your columns,

Top Programme = 
var __Client = Booking[ClientID]
var ClientProductAmount = 
        TOPN(1,
            ADDCOLUMNS(
                SUMMARIZE(
                    FILTER(Booking, Booking[ClientID] = __Client),
                    Booking[Product]
                ),
                "Amount", CALCULATE(SUM(Booking[Value]))
            ),
            [Amount]
        )
return
    MAXX(ClientProductAmount,Booking[Product])