I really enjoyed the movement and rhythm of writing with a flex pen after getting the Noodler's Ahab. Soon after, I was looking for more flexible fun so I decided to get the Pilot Falcon. I know both these aren't true flex pens but I think they are good instroductions without getting into vintage.
Pen: Pilot Falcon SF
Ink: Pilot Iroshizuku Yama-Budo
Paper: Rhodia No. 16 Grid