What's the oldest iOS product you still use daily?
Our one and only iPad in the household: iPad 3 (2012):
It's mostly used as temporary kids entertainment privilege device throughout the day. I play RR3 on it - because I like the weight better than iPhone - and the game runs absolutely fine on this old iPad. And I use it for invoice editing when I'm away from PC (Numbers app). The iPad has been thrown down the stairs, used as a tray to catch my daughters throw up when she got stomach flu (yeah), and as a skate by my 1.5 yr off son: we're talking screen-to-tile scuffing against the floor while his foot is on top pushing it along. This thing is a tank.
Also,
iPhone 5:
It is setup as a remote camera in house.... it is always ON, and it is accessable remotely any time, from anywhere. The screen stays dim, but also ON. This phone just keeps on ticking.
For those wondering: I'm using the "manything" app. I tried others - but this one is best quality and costs absolutely nothing to use one old phone as remote camera.
I know it's probably not what you were asking.
My old iPhone 5s:
Is still my favorite iOS device of all time. Best size, futureproof 64bit architecture, absolutely best design of all that is mobile. I'd use it everyday, but purchased iPhone 6s for Apple Pay (if only I knew about soon-to-be released iPhone SE -- I would never get anything bigger). I use it around the house sometimes instead of 6s just because I like the feel and size of it better. Everything is synced (cloned) between my 6s and 5s - so I can pick up any of the two and go on with normal activities (except for Apple Pay)