The best DCS Module is the aircraft you love the most. The one you would have had as a poster on the wall of youur childhood bedroom. The one you would be most excited to see at a museum.
Learning a DCS Module is a 20-80 hour time commitment. You won't be motivated to study unless you love the aircraft.
That said, if you truly don't know what to pick, try this short quiz to get a short list of modules you should trial.
Always try before you buy. Some people love the Viper and hate the Hornet, some dislike the Viper and love the Hornet. Some people like both, or neither. It's subjective.