Essentially, what the title said. Given that new Roman sites, for example, are still discovered in Europe on a relatively regular basis, presumably there must still be many undiscovered sites out in the inhospitable corners of the world. What are the best candidates?
We've barely scratched the surface of South American archaeology at this point. You think Macchu Picchu is cool? It's just the most spectacular of hundreds of sites out in the eastern quarter of the Inca Empire, many of them completely untouched by modern archaeologists. Meanwhile, the Inca only occupy less than a hundred of the last years of a timeline of empires rising and falling going back as far as the Egyptians. Imagine if all we knew of ancient Egpyt was Ptolemaic propaganda, with just a few flashes of the biggest hits before them (Let's say Akhenaten, Rameses II, and Khufu as Egyptian luminaries roughly equate to the Moche, Nazca, and Tiahuanaco). The 21st Century is going to see some amazing discoveries come out of Peru, Ecuador, and Bolivia.
this question may be worth x-posting to subs with more archaeologists, e.g. /r/AskAnthropology, /r/AskScience, or /r/Archaeology
Speaking of Tut's tomb, if you've never read Howard Carter's account of the discovery, it's worth a read.
The Kingdom of Saudi Arabia is a strong candidate for some pretty amazing early Islamic and pre-Islamic archaeology. I don't know if it would be quite as monumental or spectacular as King Tut or Angkor Wat but there are some incredibly important questions that could potentially be answered by more exploration (earliest Quran verses, the exact nature of Meccan and Medinan trade being two issues that immediately come to mind.) The Saudis are ideologically and religiously opposed to much archaeological exploration.
On the other hand that opposition also means that they are in the process of actively destroying a number of sites, particularly in Mecca and Medina where they carry out new construction in total disregard for what might lie beneath the surface.