Beng Mealea was my favorite temple of all the ones I visited on my trip. It has been consumed by the jungle and left to ruin, which to me only adds to its beauty. You can climb up and over fallen rocks and rubble. Since there are many trees, it is also much shadier than other temples.
It is quite far from town (two hours one-way by tuk-tuk) so I would recommend hiring a car for the day (although it might be pretty expensive as the tuk-tuk ride was $30). It was really dusty as well so our driver bought us surgical masks to wear and I had to close my eyes constantly. Also, it is not included with the Angkor Temples pass so you have to pay an extra $5 for a ticket. But it was definitely worth it!
Day 4: Banteay Srei, Landmine Museum, Butterfly Center, & Angkor Balloon!