OBS uses the best open source video encoding library available, x264, to encode video. However, some people might experience high CPU utilization, and other programs running on your computer might experience degraded performance while OBS is active if your settings are too high for your computer's hardware.

What Encoder should I use for streaming?

For streaming, you have to use bitrate-based rate control (CBR), and here it depends on the power of your CPU. x264 has the best quality, but takes away much CPU. Hardware encoders like NVENC and Quicksync don't use CPU but produce less quality.

Is it better to stream with x264 or NVENC?

The less work your CPU has to do, the better your games will run and the less likely you are to drop frames while streaming. But there is a slight trade-off in quality, and the reality is that x264 looks better than both NVENC and AMF when given enough power to do its job.