Be aware that a visit during sunrise brings a lot of crowds. However, you wouldn't want to miss the scenery - it's just magical. In order to avoid the heavy crowds, arrive in the afternoon, and instead enjoy sunset before the temple closes.
Following the typical weather patterns for much of Southeast Asia, the best time to visit Angkor Wat is during the dry season from late November to early April.
We got a Tuk Tuk driver from our accomodation and started at around 5am to see the sunrise over the main entrance temple. The buildings and surroundings are stuning and definitely worth seeing. Make sure you're dressed appropriately and have plenty of water with you (although both clothes and drinks can be bought from local vendors).
The site is huge, so better grab a two days ticket. The heat and humidity in a jungle environment make it a bit tired. Good and comfortable shoes with good grip to climb over several temples are highly recommended.