Some more tips:
» If your stay is going to be 2 or 3 days, you have to take into account how you are going to get around the city.
If you are going to move by taxi, tuk-tuk or with a private vehicle, the only recommendation is that the hotel is not far from the tourist areas shown on the map.
In the case of using the collective public transport, moreover, it should be located as close as possible to the MRT subway network or the BTS Skytrain.
» For a one-day visit, bear in mind the departure airport and plan carefully the sites to visit.
Depending on your schedule, the hotel will have to be located either close to the area where you will spend your leisure time (if you arrive in Bangkok at night) or close to the departure airport (if you leave the city early in the morning).
» My personal experience travelling throughout Thailand is that in general 3-star hotels must have a minimum score of 7.5 on booking.com. If you are interested in any accommodation but it does not reach this score, it is worth checking the reviews to know the reason. The 4-star hotels must exceed at least 8.0 score.