I've done a lot of travel these last few months and years and eaten lots of exotic fruit in lots of exotic places. But I haven't anywhere found papayas as good as the ones I regularly find in London. They are the jumbo papayas, they come from Brazil, they are available in most ethnic grocery stores (I get mine in Kingsbury, I've seen them in Green Valley for example at a higher price point), they retail for about £1.20 per lb which generally means about £3 per whole fruit.
I picked one up last Sunday, let it ripen during the week and started eating it over the weekend. I think this batch (the little sticker on the fruit says "formosa") is the best one yet, the flavours are immense, sweetness you wouldn't normally associate with papaya followed by a kind of counterbalancing burnt nuttiness, a wonderful overall flavour profile. Squeeze a few drops of fresh lime juice or drizzle some bottled yuzu over a few chunks and you add another wonderful complimentary flavour dimension.
The mango may be the king of fruits but the papaya is very definitely a member of the royal family :-)