Which WiFi channel to use.

Use channel 1, 6 or 11. Read on for an explanation.

Wireless routers operate on about 13 different channels depending on local regulations (only 11 in the US). The channels are spread over the frequency spectrum allocated for WiFi. Each channel represents 5MHz, however WiFi is broadcast over 20MHz in total, so a signal on channel 5 for example actually spreads over channels 4, 5, 6, half of 3 and half of 7.

If your wireless router uses a channel other than 1,6 or 11, that configuration will slow down your network as well as routers on every other channel it overlaps with.

The more networks that operate on the same channel, the more interference each one experiences and the worse the signal you will get

The problem is that wireless routers do not cooperate if they are not on the same channel. Devices on the same channel will share the bandwidth equally, but putting devices on separate overlapping channels could lower your performance by 90%, because all devices are trying to talk at the same time.

If you experience poor performance or disconnections and find a nearby device on an overlapping channel, the only way to solve this is to ask the owner to switch to either 1,6 or 11.
Stopping the interference is the only way to solve the problem.
Increasing your transmission power will have no effect.