There are several 'better' hotels that offer 'international standards' of accommodation. These tended to be booked by tour groups or people with travel agencies that planned their itineraries (my parents stayed at one of these places). These hotels are typically named after the various Temples at Angkor and can be found online easily. They will have air-conditioning and usually restaurants (but go to the street for variety).

For those on a tighter budget, you can stay at any of a number of guesthouses. Again, it is difficult to recommend one over the other. One has a nice garden, but the rooms are smaller, the other has better this or that. For the most part, they were all clean, well-run, and the families running them were very nice and willing to help you in any way they could. The rooms are spare, but most that I saw had comfortable beds and good fenestration. My room had a huge bed and private bathroom. It cost about $12 per night. If you are coming from Pnohm Penh (which is likely) you can get the name of a place from one of the guesthouses there - it will be with their affiliated guesthouse. So if your happy there you'll probably be happy in Siem Riep.

I found accomodation without a reservation at the first place I looked. That was probably the easiest part of getting settled from leaving PP.

I would not recommend one place to stay over another.