First of all, I wanted to thank @pjs for sharing his experience with me. I contacted his driver but the quote was too high for me traveling solo. I ended up using Siem Reap Shuttle for their Angor Sunrise and Koh Ker tour. I thought they were fine. They have a ticketing office in downtown Siem Reap just across from the Park Hyatt and a main office a bit further out.

For the Angor sunrise I was picked up in a small van and we only had 6 of us so everything was done quickly. The tour was fine, The driver and guide were very polite and provided iced water and towel. It was disappointing that we didn't see the sunrise because of cloudy sky, apparently it can be cloudy during summer for both Sunrise and Sunset.

For the Koh Ker tour, I was picked up from hotel and then transferred to their main office which is also a coffee shop. This is also their collecting point for other travelers for the same tour. I waited for may be half an hour before we went on our tour. It was a very small group with just 4 of us - 2 Japanese women and a Korean guy. Again, the driver and guide were very polite and the flow was fine,

They told me they have 14 guides and also do private tours and airport pickup.