HDP
Definitive answer and one you will probably not sell for sometime due to well.. Everything about it. Size, total versatility, good headamp, DAC is good enough until you hit Benchamark Dac1 or 1k DAC's, huge performer for bang for buck.
Pre-amp function which I find VERY useful. Also drives most everything quite well with satisfying slam. Not ultimate authority. You need tubes for that, but the transparency is very likable and you'll probably never sell it due to how functional it is..
HDP is a performer... I feel I should reccomend you other amp/dac's, (If nothing else to keep the market competitive) but this is ultimately the #1 choice under $500. Yulong D100 is also good choice depending on your needs, but not as small and not as much connectibility.
The DAC on Yulong D100 is pretty much the best DAC under $1000. That is widely accepted, but with the D100, you're basically buying a really nice DAC + an amp tacked on... (You'll likely want a better amp, if not need one.)
The DAC in HDP competes, but doesn't quite live up to D100 DAC, but other than the HDP is the better buy. Size/portability, more functionality. It doesn't slow down for anything.
Not hating on the Yuolong, it's a great product too and if you can find one used you're in a good place. But as an overall product, HDP takes the cake.
Edit: Slight grammar correction: