For my modern consoles, I use one of
these. It works well enough, and even automatically switches to whatever device is turned on and plugged in. Unfortunately it's prone to interference from various appliances and the video, audio, or both can cut out from time to time which is super annoying.
On my CRT TV for my older consoles, I just use some cheap generic brand 3 input A/V splitter. It works fine, no degradation in signal quality from what I've noticed and it has S-video support. It's not really enough anymore and I'll have to get another one soon though.