The ruined temples of (and near) Angkor Wat are on my short list of the most amazing places I've ever been in the world.
Angkor Wat is the largest and most famous of the Angkor temples, and it's completely breathtaking. It is a massive tiered structure crowned by five towers that kind of look like enormous beehives. When you visit, you can pretty much just wander around the temple, which was built to the Hindu god, Vishnu. Angkor Wat is surrounded by a moat and an exterior wall. Inside there are some really steep (and a little scary, considering most don't have railings) steps up to the different levels of the temple. The interior and exterior walls of the temple are covered inside and out with literally thousands of bas-reliefs and carvings.
Angkor Wat is the largest and most iconic temple in the area, but there are other fabulous and fascinating ruins to see, aside from the big temple. Angkor Thom (Big Angkor) is the royal city with walls and a moat. Inside are many other sites worth visiting. There are five gates to the city, one for each cardinal direction, plus, the "victory gate" which leads to the royal palace area.
One of the incredible sites inside the city is the Bayon, with giant stone faces carved onto large towers. There are other carvings with scenes of everyday life, along with scenes of battle. It's really fun to just wander around and explore these ruins--there are tons of little rooms and hidden alcoves. Nearby, the Elephant Terrace is also worth a visit--it's basically a long road (apparently it was a parade viewing platform or something) which has loads of carved elephants and lions.
But of all of the sites at Angkor, my personal favorite was Ta Phrom, which you might recognize from movies like Tomb Raider or Indiana Jones and the Temple of Doom. Ta Phrom hasn't been restored, and the jungle has reclaimed the temple, with huge tree trunks growing out of the ruins and vines snaking around the stonework. Apparently, the enormous trees and vines are part of the reason this part hasn't been restored--the jungle is the only thing holding part of these structures together.
There's no way to see all of these stunning ruins in one day. Even two days would feel rushed, if you ask me. It's worth it to stay in nearby Siem Riep for a couple of days, and especially to build in a visit to Phenom Bakeng, to see the incredible sunset over the temple. It'll be impossible to ever forget the experience.