I haven't been to Jeju, so are the lava tubes really pretty spectacular? I've been in lava tubes at Hawaii Volcanoes National Park, but I don't know how they compare to Jeju.

I think we should leave Jeju and Giant's Causeway in the maybe-category, as they represent specific features and to distinguish them from the more well-rounded other 5 that are definitely a Y.

What about the remaining italic ones? Are they in the same league?
Macquarie Island - " being the only place on earth where rocks from the earth's mantle are being actively exposed above sea level" might be of some specific interest. A maybe?
Mount Etna - "This exceptional volcanic activity has been documented by humans for at least 2,700 years – making it one of the world's longest documented records of historical volcanism" -> we already have the nearby Isole Eolie, also with long history. I would say N
Pitons Management Area - two eroded remnants of lava domes, "the most iconic sight in all of St. Lucia" -> not really of global significance N
Pitons of Reunion - it's about a 'visually striking landscape' of Pitons, cirques and remparts; don't know how visually striking it is, enough to call it iconic? A maybe at most.

That reminds me very much of Gros Morne, which also has an exposed section of the earth's mantle (though maybe not being actively exposed?). Both sites have ties to plate tectonics, but if I were explaining plate tectonics to a little green man, I'd probably start out with Hawaii Volcanoes (and the Hawaiian island chain) and the Swiss Tectonic Arena Sardona. Although Macquarie Island wouldn't make it on my list, maybe others would find it more significant.

This is the challenge with sensitive cave habitats. Only 1 of the 3 caves systems are visitable, the one open to the public is certainly massive, striking, and unique, However, few would call it "pretty", the two other caves are extremely fragile and include a rare underground alkaline lake and a rare limestone cave fused with a lava tube system, quite spectacular. Based on the nomination file, it compares well with Hawaii Volcanoes NP's Lava Tubes and is generally considered more significant and outstanding as a Lava Tube system.

What makes Jeju special is the breadth of the nomination. Habitat, Lava Tube System, Volcanic features (tuff cone, crater lake, oreums) Nevertheless, based on the truly exceptional qualities of the iconic sites already mentioned, Jeju may not make the cut, I just wanted to share some of the exceptional qualities of this natural site.

"Jeju Volcanic Island and Lava Tubes is a coherent serial property comprising three components. The unequalled quality of the Geomunoreum lava tube system and the exhibition of diverse and accessible volcanic features in the other two components demonstrate a distinctive and important contribution to the understanding of global volcanism." Advisory Body

"The flora at the Mt. Halla National Park is unique. 1,565 vascular plant species have been recorded in the area thus far and is the highest number of plants in any mountain, 33 which are endemic to the island. Unlike most other Korean mountain environments, Hallsan has a unique vertical distribution of plants in three different zones: the subtropic, temperate, and frigid zones." wiki

The summit/crater lake at Hallasan NP and the Ilchulbong Tuff cone certainly stand out as iconic within the East Asia sphere.

Regarding Macquarie Island.
The Antarctic/sub-Antarctic region is not as well represented on the full list as it should be - both because of the special situation regarding the continent and also because UK hasn't come forward with the real star - S Georgia.
We have I think 3 to choose from - this , NZ subantarctic and Heard/Mcdonald. So 1,2 or3 of these? Which is the best and do the others offer anything extra which is special? is Macquarie's volcanic pedigree special enough?Heard/McDonald offer this too.

The AB ev's do not really help:
- Macquarie has what the other two have PLUS "being the only place on earth where rocks from the earth's mantle are being actively exposed above sea level"
- however "Macquarie is inferior in biological and in scenic terms" to NZ subantarctic (and suggested is to combine both into one WHS); Macquarie als has naval base
- Heard & McDonald don't have an AB ev on the Unesco website. It seems superior to the other two in volcanic features ("It noted that this site is the only volcanically active sub-Antarctic island and illustrates ongoing geomorphic processes and glacial dynamics in the coastal and submarine environment and sub-Antarctic flora and fauna, with no record of alien species."), and has never seen human impact. I also like that it is essentially the hardest to reach WHS (in days, see http://www.worldheritagesite.org/tags/tag1214.html)

The Macquarie AB is certainly interesting - IUCN didn't really feel that it's geological uniqueness was enough and hinted that they would have preferred it to be linked with NZ islands.
I think there is little doubt that HIMI is more "Antarctic" and fully wipes it face on faunal matters.
Is this rather niche exposure of mantle enough to give Macquarie the edge??
I feel our little green man would prefer a trip to HIMI if we weren't going to take him to Antarctica.

We'd put him on a 25 day cruise!

And what about Tongariro? Included in this Category later.

I would not say so, see also https://en.wikipedia.org/wiki/Ophiolite

For me the Volcanoes of Kamchatka would be the best representative for the Pacific Ring of Fire, and would supersede Tongariro.

Now, if New Zealand had decided to put forward a tentative nomination for Rotorua and the Waimangu Volcanic Rift Valley, that might be more interesting because of geothermal activity. The rift valley used to be home to the Pink and White Terraces, one of New Zealand's biggest tourist attractions in the 19th century, before they were destroyed by the Mt. Tarawera eruption in 1886. The rift valley is now home to Frying Pan Lake, the largest hot spring in the world. Even if Rotorua and the Waimangu Volcanic Rift Valley were ever to become a World Heritage Site, though, I still think Yellowstone would be the best choice on the list to represent geothermal activity.

Summarizing again. I think we might be there now:

Giant's Causeway - basalt formation formed by volcanic activity at a fine location
Hawaii Volcanoes - best example of volcanic island-building
Heard and McDonald Islands - representing the sub-Antarctic, no record of alien flora or fauna species and volcanically active
Hierapolis-Pamukkale
Isole Eolie - Significant in the early study of volcanism
Jeju - for its Lava Tubes
Macquarie Island
Mount Etna
Pitons Management Area
Pitons of Reunion
Surtsey
Teide National Park - Best example of a Stratovolcano
Ujung Kulon National Park
Volcanoes of Kamchatka - representing the Pacific Ring of Fire
Yellowstone - best variety of geothermal features on the World Heritage list

My selection:

Hawaii Volcanoes
Hierapolis-Pamukkale
Jeju
Mount Etna
Ujung Kulon National Park
Yellowstone

There are three in support of Reunion which is currently the only example of volcanic plugs.

Updating the list:

Giant's Causeway - basalt formation formed by volcanic activity at a fine location
Hawaii Volcanoes - best example of volcanic island-building
Heard and McDonald Islands - representing the sub-Antarctic, no record of alien flora or fauna species and volcanically active
Hierapolis-Pamukkale - geothermal features
Isole Eolie - Significant in the early study of volcanism
Jeju - for its Lava Tubes
Macquarie Island
Mount Etna - the definitive volcano
Pitons Management Area
Pitons of Reunion - volcanic plugs
Surtsey
Teide National Park - Best example of a Stratovolcano
Tongariro
Ujung Kulon National Park - volcanic eruption (Krakatau)
Volcanoes of Kamchatka - representing the Pacific Ring of Fire
Yellowstone - best variety of geothermal features on the World Heritage list