My perfect flashlight would have an aluminum body.
The battery holder would be attached to a sealed switch switch and the LED bulb using a slide in connector with contacts made of iridium. The battery holder could be a simple flat plastic rectangle like many battery holders are. It would hold a single battery. The end of the batter holder would have two short prongs hidden inside a protective hood. The battery box would slide into the flashlight from the bottom. I would think a 26650 or an 18650 Li-ion rechargeable. The battery box could slide into a charger. A second battery box that held two AA or AAA could be included.
The battery would drop in between two flat spring steel made from iridium. Nothing to do about the battery contacts except include a piece of scotchbrite in the body of the flashlight. The AA or AAA battery box could use cheap wire springs, but, flat iridium springs would work :-)
The LED would be able to use the body of the flashlight to dissipate any heat.
The bottom of the flashlight would be a slip on rubber end cap that would make the flashlight water resistant. The end cap could be something like the slip on tips for a cane or a walker, although it could be rectangular rather than round. Round is easier There would be a grove for an o-ring near the bottom of the body of the light. The water resistant seal would be between both the rubber of the end cap and the o-ring.
The switch would be in the body of the light near the bulb. It would have a cover of some kind so it could not be turned on automatically. If the flashlight were round it could have a friction twist cover that used o-rings to provide the friction.
That should make a ridiculously expensive flashlight that no one would buy because they wouldn't understand how cool it really is.