The easier way to reach Angkor is flying to Siem Reap International Airport. The airport is located about 8 km out of Siem Reap and has been rebuilt recently. It looks new and cozy, with modern design. Currently the airport can manage 1.5 million passengers per year. At the moment, there are 8 different companies that fly to Siem Reap, and they are: Bangkok Airways, B4 Far Eastern Air Transport, Lao Airlines, Malaysia Airlines, Siem Reap Airways, Silk Air, Vietnam Airlines and Korean Ai. So from Siem Reap you can reach directly Thailand, Vietnam, Laos, Korea, Malaysia, and Singapore. Considering that most of the arrival airports are big intercontinental hubs (Bangkok, Singapore, Kuala Lumpur) we can say that Siem Reap is pretty good interconnected with the world. Of course there are many flights to/from Phnom Penh. Consider also that charter flights arrive here sometimes too.
Passengers are invited to check in at least 2 hours before departure and it closes 40 minutes before. You can board up to 7 kg of hand baggage, so more than the usual 5. The whole airport is no smoking area except one bar that has a room for smokers. There are Hotspots in the airport, so you can connect to internet while you wait for your flight.
To enter in Cambodia, a valid passport and a VISA are required. Visas can be obtained at Siem Reap International airport upon arrival. It costs 20 UDS for private travelers, or 25 UDS for business travelers. A photo is required to get a VISA, but if don't have one, you can simply pay a little fine and you'll get your VISA the same (the fine is about 2 USD). Length of stay is 30 days including arrival date. If you take a domestic fly, the tax is currently 6 USD. The airport has a currency exchange booth, and all the major currencies are accepted. So, don't worry if you arrive without dollars or riels.
The taxi to Siem Reap costs 5 USD and it takes 15-20 minutes. There is a coop of taxi drivers, most of them are young guys, friendly and in love with their land. It's a good idea to arrange something with them for your visit to Angkor or Tonle Sap for many good reasons: the guys are nice and friendly; they know really good Angkor and its temples and they are often wonderful guides; they know how to get the best for the right price and how to avoid to be scammed; they are honest and reliable. So don't lose this chance, and book your taxi/guide as soon as you arrive. If you don't have and hotel already booked, ask them. They will bring you to a place that is in line with your budget. The only thing is, their English is far from perfect: be patient and tolerant about this!