the DPMS panther is a good one, but I have sold mine due to unavailability of spare parts. For my purposes, the FN FAL is the best option. There are several variants of it such as the springfield SAR 4800 etc, but in all, there are enough of them out there that long term storage/shooting servicability are addressed. My opinion is that if you can't get parts, it's not worth having. No matter how tough a firearm is, they are all suceptable to breakage. Some are more durable than others such as the FN or glocks, but at the end of the day, the right corrosion or impact conditions can break any of them.
It's never going to be as good as it is today again when it comes to parts and firearms availability. Every firearm I have has a complete set of spare parts with the sole exception of the receiver for that reason. If you are still legal to shoot these firearms in 10 years, I seriously doubt you will be legal to buy parts or the firearm in the same time frame.
Regards,
Gerald