Commercial Aircraft (payware) offer some of the most realistic experiences but with a steep learning curve. Type Ratings are modeled after their real-world counterparts to teach the ins-and-outs of every system on-board.
Airports all have their nuances and many of the large airports can be intimidating to operate within. Airport Certifications help familiarize the airport and its procedures so future visits can go smoothly.
With a low barrier to entry, joining an online network for a flight provides both real aircraft traffic and air traffic services from trained controllers both of which helps increase the realism on every flight.