Which Ubuntu release should I use?

If you are new to Ubuntu; always go with the LTS. as a general rule, LTS releases are what people should install. 19.10 is an exception to that rule because it’s just that good. an added bonus is the next release in April will be LTS and you can upgrade from 19.10 to 20.04 then tell your system to stay on LTS releases.

Which Ubuntu version is best for beginners?

Linux Mint is arguably the best Ubuntu-based Linux distribution suitable for beginners.