Traveling from Phnom Penh to Siem Reap is very easy. I took a bus between the two, but you can also fly or even taxi between these two cities for reasonable prices
Personally I think bus/van is the best option as it's very budget friendly and it doesn't take too long. Flying is also great if you're on a tight schedule
There is currently no train service between Phnom Penh and Siem Reap, much to my disappointment
Bus/van from Phnom Penh to Siem Reap:
Bus and/or van is probably the most popular way for locals to get between these two cities as it's extremely affordable
Prices are all between $9 USD - $12 USD
Travel time on bus is mostly between 5hr30min - 6hr. However, given the traffic, it's possible to take up to 7 hours
There are a LOT of bus and van companies running from as early as 7am to as late as 11:30pm
Just to list a few I came across during my research:
Giant Ibis: website, Tripadvisor (rated 4/5) <- this is the one I took, was very comfortable
There is no central bus station that serve all these buses, each bus company has their own pick up and drop off area
I recommend using booking companies like 12Go (https://12go.asia) or BookMeBus (https://bookmebus.com) to book tickets. I used 12Go as I've used it before elsewhere in Southeast Asia, but BookMeBus had a lot more options and departure times. Both of these websites show you exactly where you will get picked up and dropped off and you can book with international credit card
Flying from Phnom Penh to Siem Reap:
Flying is the quickest way to travel between PP and SR
Flying time is only 1 hour. Including the time to get to/from the airport and such, total travel time is 3-4 hours
There are currently 3 direct flights daily (10:05am, 1:55pm, and 7:45pm), offered by Cambodia Angkor Air, which is the national airline of the country
Fare is $45 USD each way
Because Cambodia Angkor Air is the only airline flying this route, I recommend booking directly with them through their website
There are many different options to get to the airport in Phnom Penh, but taxi or train is probably the best. In Siem Reap, taxi is the best way to get from the airport to the city
Taxi from Phnom Penh to Siem Reap:
An environmentally bad way to travel long distance, but it is a comfortable way to travel between PP and SR
It's not really a "taxi" in the traditional sense as in you hail it on the street. It's really a private transfer service that you book in advance
It is the most expensive way to travel between these two cities, costing $75-$95 USD each way per 4-seater car. But if you have a small group, this can actually be very reasonably priced
They usually use SUV for this service, so it's actually very comfortable
Travel time is 5-7 hours each way, similar to bus
You will need to book a taxi in advance. Most hotels will happily do it for you in Phnom Penh, but you can also book ahead online:
BookMeBus (https://bookmebus.com): they're not a taxi company, but just a booking portal. They mainly book buses but there is a section to book private taxis