Heathrow
Travelling fairly frequently means that I often see different airports and varying degrees of shopping offerings at the airport.
For many, it is an opportunity to pick up some designer items at tax free prices, or an opportunity to purchase some last minute presents for people you might be visiting.
For me, it's both. The selection at T3 is quite limited and I would recommend T4 or T5 for a wider selection of designer items.
Duty free is such a life saver! Some really nice stuff to be had