I'd say London Control is the best ATC simulation you can get, SSS the best freeware.
If you want to talk to real people, VATSIM and IVAO should be your choice, which of the two depends on personal taste really.
Both are pretty professional, IVAO seems to have more attendance in Europe (especially the south of it), while VATSIM attracts more participants in the US and some parts of Europe like the UK or Germany.
Regards,
Robert