VATSim is great!
AT larger airports and during published events it gets a little hectic so for a rookie I would advise against flying there as controllers don’t have time to be patient.
DEN,BOS,SFO and in Europe all London airports and mostly Germany,France, the Netherlands, Switzerland, Austria.
As for learners I would really strongly suggest ANY Canadian location as they are just like Canadians are, eh. LGA GND and TWR are also always super-helpful and understanding.
If you want fully controlled airspaces, check out the UK or Germany. Crazy good coverage there. Or the US Northeast.