Nano 4th Generation
I have a 16GB 4th Generation and use it continually. It's a superb device and lives at the intersections of good form factor, utility and service ability.
I want to stress that the 4th Gen is more repairable than 5th Generation. The 5th Gen has a speaker and camera crammed into the package that will make long term repairs much harder.
The 4th Gen has an 'appropriate' feature set. Unlike the 5th Gen it doesn't have a video camera, or radio built in.
Apple has provided a feature comparison chart between the 4th gen and the 5th Gen Nano here:
The 4th Gen I am able to make work again in 7 out 8 instances when it's been sent for water immersion. The LCD can be replaced if cracked.
Over the long term unless you carefully charge and discharge the battery, you will have the same problem that exists with the Nano 2nd Gen. Since the outer case/housing of the 4th gen is an extruded eliptical tube as the battery ages and its chemistry unbinds the will inflate taking up cintually more space inside the housing. There will be a point when the battery will occupy so much space inside the device that I won't be able to slide the inner workings out of the case without damaging something. I imagine that will be around 2013.
You won't be able to find one of these new anywhere. The second best chance will be to get a Nano 5th Gen off the Apple website here:
They sometimes have 8GB Nano 5th Gen models for $99
The 5th Gen is similar to the 4th Gen but everything is more crowded inside it because of speaker and radio. Do Not get a 6th Gen.