I guess the 'best' one is the one that most suits your needs without having to shell out a zillion squid. I have a Sony X-series and a Samsung Galaxy S media player which is soon to be rooted to enable me to install voodoo sound. I currently prefer the X-series but both have their pros and cons.
The Ibasso DX100 is arguably the current king of Daps with its great spec list and capabilities,
The information from iBasso:
24Bit/192KHz Reference Digital Audio Player
- Android 2.3 OS With Custom Audio Player Software
- Support up to 24Bit/192kHz Bit for Bit Decoding
- ES9018 32Bit DAC Chip
- Built-in +/-8.5V Headphone AMP
- 3.75" Capacitive Touch Screen
- Up to 24Bit/192 Optical/Mini Coaxial Output
- 3.5mm Headphone Output, 6.3mm Headphone Output, and Line Out
- 256-Steps Digital Volume Control
- 64G Onboard Flash
- Support up to 32G External MicroSD
- 3-Setting Gain Switch
- SRC Function
- Slow Roll-off/ Sharp Roll-off Digital Filter
- Support Wifi, Bluetooth
- Solid Case Made of Magnesium Alloy and Aluminum Alloy
- Audio Formats Supported: APE, FLAC, WAV, WMA, AAC, ALAC, OGG, MP3
- Come With Micro USB Cable, Coaxial Cable, and AC Adapter
- One Year Warranty and Ten Years Free Labor
Specifications:
Power Source:Built-in 2000mAh 8.4V Li-polymer Battery pack or external power supply
Frequency Response: 20Hz-20KHz +0.1/-0.25dB
Signal to Noise Ratio:-116dB
Crosstalk: 1KHz0dB > -100dB, 20KHz odB = -82dB
THD+N: 0.002%
IMD: 0.0034%
Gain and Output Power: 0dB=2V rms (125mW/32ohm)
+3dB = 2.8V rms (245mW/32ohm)
+8.5dB = 5.0V rms (83mW/300ohm)
Battery Life: 72Hours (stand-by) or 7Hours (play music)
Battery Charge Time: 4Hours
External Power supply: 12V DC
Recommended Headphone Impedance: 8~600Ω
Dimension: 2.83W x 4.65L x 0.93H (inch)
71.8W x 118L x 27.5H (mm)
Weight: 265g or 9.3oz
http://www.head-fi.org/t/592076/ibasso-dx100-true-24-bit-for-bit-reviews-impressions-inst-downloads-imgs-video-pg-1
I couldn't justify spending that much on a player just for casual listening with my TF10's as I'd never use it to its full potential, but thats just me.