Nano 5th Generation
This is the model where Apple decided to throw everything in and see what happened. The Nano 5th Gen is feature packed. A lot of things very few will ever need or shouldn't rely on.
One example.... the built-in FM radio that relies the earbud wire for an anntenae. Oh, and it's a radio TiVo (Apple calls this "Live Pause") recording the radio and buffering 15 minutes of the programming. If you have an iPod and it's already loaded with things you want to hear why are you going to need a radio? I'll tell you why... if you're at the gym and you want to watch TV while on a treadmill to hear the TV they have you tune into an FM radio. So that feature I'm sure was included only for people who are treadmill/stepper TV watchers at the gym. For everyone else it's a "no sale".
The 5th Gen also has a built in speaker. Apple borrowed the piezo tranducer concept/technology from the Touch model which has had a speaker for the past 3 years. The speaker in the 5th Gen Nano is so tinny that it's awful. There are also many instances I've experienced where the speaker can't be shut off while the iPod is in use.
There is a camera and microphone in the 5th Gen that take basically really poor quality photos or poor quality videos. I almost think you would have a better memory by remembering it without using the camera.
Because the speaker and the camera are jammed into the device, it makes safe dis-assembly impossible for the amateur. It also guarantees once the battery starts to expand from failure just a little bit, there will be no way into the device safely for repair of any sort.
These features, the camera, radio and speaker I could see living without and why I prefer the 4th Gen Nano personally.
My order of Nano liking is
1) 4th, like the most
2) 3rd, good serviceable 2nd choice
3) 2nd, reliable and appropriate
4) 5th, can do the job, not as serviceable
5) 1st, not good for anything anymore and under-engineered when first released
6) 6th, an abomination