We cannot recommend Angkor Cycling Tours enough. We did two full day tours a few days apart: Angkor Region and Rainforest Tour (35km, 8 hours), and East Corner of Siem Reap Temple and Countryside (45 km, 7.5 hours).
We had Seng as our guide for both tours. He was always smiling, knowledgeable, with a good level of English. He made our tours thoroughly enjoyable and he is passionate about his job which makes a difference. In particular the East tour, Seng has a lot of extra knowledge about the locals as he comes from that area. His pace of cycling was perfect.
You cannot experience Siem Reap like this by tuk tuk. The cycling trails were beautiful and we went to a lot of hidden temples in the forest only accessible by bicycle where we were the only people there. You get to see way more than what is simply listed in the "what's included" bit. The East tour is less popular but if you have time it is definitely worth it. With this review are a few photos of some places you are not going to see by tuk tuk.
The experience was amazing value for money, with lunch included (our lunch on the first tour would have been $17 as it was at one of the more touristy temples), and unlimited water and Seng was always checking if we needed more. Also we had quite a few stops for coconuts, soft drinks, and coffee one time. We interracted with the locals in a way you cannot on a drive-by tour.
The East tour was a little harder than the first with more distance and bumper terrain, but if you have a moderate fitness level and don't mind a sore bottom it is well worth it. You don't need to be a cyclist.
Thank you Seng for making our trip to Siem Reap so special!