Too many metrics which could be used to define "Best".
I currently own a large collection of handhelds covering most produced, the original Gameboy is a pig to play now, the Pocket or Colour are useless without a back-light, the Gameboy Light screen is still rubbish, the Neo Geo Pocket and Pocket Colour are hard to play but i like the controls, the Lynx and Game Gear standard screens suck and suck battery life too, stuff like the Wonderswan isn't even in for a shout.
Gameboy Advanced, Micro and SP start to look better than the above, never got along with the DS, PSP is OK but dislike the controls.
The Switch is without a doubt better than all the above, but how do you define best? Most powerful?
On reflection i think the "best ever" has to be a Nintendo handheld, the Gameboy Advanced SP got a lot of things right (despite being a hard shape to hold).