Wat Thmei is a working Buddhist temple that also holds one of Siem Reap's most accessible memorials to the Khmer Rouge period. A glass-sided stupa contains bones and skulls of victims from nearby killing fields, making the visit sobering in a city otherwise dominated by temple itineraries, restaurants, and tour desks.
The memorial is modest, and that modesty is part of its force. Come respectfully, keep your voice down, and give the monks and local worshippers space. It is best treated as a short, serious stop that adds modern Cambodian history to the Angkor-focused circuit.