Of the many I recall, the most spectacular by far is the Tongariro Crossing in North Island, New Zealand. It is an unforgettable 8-hour hike over the volcanic heart of the island.
Deposited by minibus early in the morning, you ascend up the steep western flank of the National Park to the plateau below Mount Ngauruhoe - everyone's image of what a volcano should be. Some of the keenest - or most foolhardy - give in to the temptation to divert from the Crossing to scramble up to the top of its cone. I was no so tempted, but preferred to stay below with my camera.
Then you then you turn towards Mount Tongariro and ascend to follow round the narrow edge of the Red Crater - seen here, top right - before descending a narrow ridge towards the Emerald Lakes. The ground beneath your feet is hot. Steam hisses from the rocks. You do not even think of trying to leave the path. You sense you are on holy ground, or utterly threatening and unstable ground, whichever way you choose to see it.
People descending the ridge on the right become no more than a column of ants.
On the way down, you just have to keep turning to look back.
Anyone keen on hiking – and reasonably fit – should try this walk at least once in their life; I am unlikely ever to do it again, but it is a hike I shall never forget.