Emulator on a Handheld

Hello,

for many years i had a PSP (I dont know what Type) and it was very easy to play some Emulators on it. I only needed to install a program, start it and with it the emulators works.

Know I want to play some old GB, NES, SNES Games with a handheld to but i dont know how any more and which one is the best.

I have a New 3DS and a Playstation Vita. Maybe i buy a old PSP but which one? Ore someting else like Expiria Play?

Which Handheld is the easyiest and best to emulate? Oh and not that expensive ;)

Thanks!