iPhone 5. Not the greatest, but still, it's redonk.
I had it on a 4s. It worked, but was a little wonky. Make sure you are on the latest, it got better. Then I got a the next gen phone.
I think 9 doesn't bring a lot of new features, more bug fixes and refinements. If 8 is that bad 9 might be worth a try after a couple revisions.
I'm on the latest 8, but it probably can't make anything worse to try 9 to see if it fixes anything.
Good luck. I was kinda mad at 8 on the 4s initially.. really took any snappy performance and pleasure out of the device. I'm hoping 9 is like the "snow leopard" of iOS releases, very stable and not a lot of flash.
odd - I've got ios8 on my 5 and no problems. Something odd up perhaps.
Probably one of the ones Apple wants you to throw away after two years.