Siem Reap is a small town arranged around one road to the temples, so the choice is how close you sleep to Pub Street and how early you can get out to Angkor.
Old Market / Pub Street quarter
The block between Psar Chas and the river, holding the bars, the night markets and most of the cheap beds. Closest to everything and loud until well after midnight — fine if you are going out, hard if you are up at 4:30 for sunrise.
Wat Bo — the east bank
Across the river from the Old Market: guesthouses and small hotels along quieter lanes, ten minutes' walk to Pub Street when you want it. The usual compromise for anyone staying more than two nights.
Charles de Gaulle road, towards the temples
The strip running northwest past the Angkor National Museum and the ticket office towards the park gate — larger hotels, more space, and the shortest pre-dawn drive to Angkor Wat. You will be taking a tuk-tuk to dinner.
Sala Kamreuk and the southern lanes
The residential edge south of the centre, where rooms are cheapest and the town stops being a tourist town after about two streets. Best if you are staying a week and want a market rather than a rooftop bar.