I am using an iPhone 4S, 64GB, black, unlocked. I switched to AT&T's no-contract plan and am going to keep using the 4S until I can't take it anymore. It started to feel slow in later 7.0.x versions, but that may have something to do with my wife getting a 5S

7.1 improved performance enough that I'm happy with the 4S again. The Facebook app is the only thing that is annoyingly slow, but it's not extremely performant on the 5S either.
I can't go back to using less than 64GB, I'm buying an unlocked phone next time, and I always get AppleCare+ on my iPhones, which means upgrading is expensive, so I'm putting that off as long as I can.
I still have all my old iPhones and iPod touch:
I use my 1st-gen 16GB iPhone in my car as a glorified iPod. I leave it in the car, and I have it in airplane mode and battery lasts long enough between drives.
My 3GS is a dedicated music remote used for: iTunes Remote, Spotify w/ Airplay, Pandora w/ Chromecast, etc. I have the screen set to stay on constantly; it sits in a dock most of the time.
I let my 1-yr-old son play with the 1st-gen 8GB iPod touch. And I have a 16GB 3G that is a spare in case the one in the car dies or gets stolen.