My personal preferences would be for any of the following, I sure many would disagree:
Cowon D2 or S9
Sansa Clip / Clip+
Sony X Series
In my mind I'd avoid the HiFiMAN or Amp3 on accounts of both are too new and seem to be full of glitches, you only have to read the various threads here to figure that one out - Sure it's got it's champions, but it seems to be the same old people trying to convince us all that they're perfect devices.
I've owned a D2 and can confirm it'll drive most headphones ease, I dropped my and damaged it, I don't have anything negative to say about the D2- It's a very capable great sounding device and worthy of the praise it gets here on Head-Fi.
I've listened to a Sony X Series and was really impressed with the SQ.
As for the Sansa Clip, I've got an 8gb Clip (not Clip+) and I'll never part with it, it's by far the best sounding device I currently own - Even if it does look as if it was build by Fisher-Price - Superb sounding and great value for money.
However I'm currently also using a 4th gen iPod Nano and to be honest I can't really find anything to fault about it. Yeah it'll a little quite due to the EU cap, but sounds great amped.
Bottom line - I think that If I had the money I'd go out an purchase a Sony X Series tomorrow.