What channel is best for my router?

If there are no other networks nearby, then the channel is largely unimportant. 5 GHz is potentially faster, but has shorter range than 2.4 GHz so the distance between the router and your device(s), may affect your choice of which band to use.

Unless you have a very fast ISP connection, then the WiFi on either band should be faster than the ISP connection even with 20 MHz channels. However, if your are moving large files from one machine to another on your internal network, then 40 MHz operation (or even 80 MHz on 5 GHz) may be an advantage. Note: that both the router and the device must support the wider frequency operation to take advantage of it. Many devices only support 20 MHz channels.

If you have any fixed devices near the router, such as a desktop PC or a games console, consider using Ethernet. WiFi is a shared interface that allows just one packet at a time to OR from any device connected to the particular network. Ethernet provides dedicated connections into the router and allows packets to go both ways at the same time.

Typically, with 20 MHz channel operation, you should allow a minimum of 4 channels between your channel and another one. This is because the channels are 5 MHz apart but occupy 20 MHz of spectrum so two channels closer than 4 apart will overlap. Although very unlikely, if you do use the original 802.11b setting, then you should allow 5 channel spacing as 802.11b uses over 20 MHz of spectrum.

I hope this helps.

You need a WIFI analyzer app. Just choose channels that have spacing from detected WIFI channels.