Well the British Monarchy website says ..... "The Order of the Garter is the most senior and the oldest British Order of Chivalry and was founded by Edward III in 1348. " Wikipedia says its the most prestigious. It's an order of chivalry and it can be granted to anybody of any status. Also once a Knight of the Garter you get letters after your name - KG.
Whereas the Family order is only given to members of her own family. And the holders of it don't get any titles, letters or precedence.
There is also the Victorian Order for people who have personally served Her Majesty. Again you get letters after your name for it.
So as we say - you pays your money and you take your choice. Meaning it all depends on how you define 'highest' as to which honour you might choose to be at the top.