We used Synon on a couple of days and liked him. Prices seemed reasonable compared to the other quotes I got, including from Darith Touch and another fellow named Sophen I found through TA. Synon charged us $30 for the grand circuit and another $10 for including Banteay Srei, the land mine museum, and Banteay Samre on the same day.

Sophen was chatty and our favorite driver, but his pricing was meaningfully above everyone else's apart from the first day when he picked us up from the airport and took us to the Rolous group and the floating village and a little bit around town for $40, so we didn't use him on subsequent days. Other than price though, we would recommend him.

We mostly used a tuk tuk within Siem Reap and only used a car for the far away temples (Banteay Srei, Beng Mealea, Koh Ker, and Grand Circuit, though the first and last one is also doable by tuk tuk) and the airport. For tuk tuks, we paid $15 for the whole day and $20 for a full day on the day we went to Angkor at sunrise. We arranged this tuk tuk through Sophen - his 'brother' Vi Sa drove us around and was fine - but presumably you can arrange one off the street.

Also found Siem Reap Shuttle's group tour to be perfectly adequate for the day I took a solo trip to Koh Ker and Beng Mealea without my +1. A lot cheaper even for a private car than Sophen, Synon, and Darith and since their group tours were fine (didn't feel rushed; half-decent guide), I presume the private cars would be too.

Lastly, this was the first time I saw it, but the restaurants near the temples with printed menus have negotiable prices! They will start to discount if you walk away to check out the competition!