as I see there are two popular ffmpeg and avconv, for example what uses youtube or other popular platforms for converting one video to multiple resolutions, subtitles and audio tracks etc...are these libraries okay or is there a better solution?
You have answered your own question on this one, ffmpeg is the de facto standard for this type of work, there really isn't any reason to use anything else. It's free, fast, works on almost any imaginable platform, and can be easily integrated into almost any workflow because of how many ways it can be called. You can even use it to apply special filters.
If you aren't comfortable using the command line, there are many GUI interfaces available that will give you a nice interface for using ffmpeg. One good example is myffmpeg but there are many others.
If your goal is to always convert using the same settings though, the absolute best way to achieve this is to create a simple ffmpeg batch/shell script (depend on which OS you're using) that you simply run that script and it does its thing - no effort once it's set up the first time.