HxC SD, because it supports almost every kind of disk possible (and works with almost every machine, like Amiga, Amstrad CPC, some keyboards...). The downside: it's expensive. Also, keep in mind that HxC USB versions lacks write support, and HxC slim lacks the two line display (that helps you to select the right disk image).
I've heard some nice things about Gotek floppy emulators. They're not as flexible as the HxC, but they would work with most PC disk images. Also, it can be adapted with some firmwares to work with disk images from other systems (there is an Amiga firmware, and a CPC one in the works).
And, by far the worst emulators: those unbranded chinese emulators. They only support 3.5 floppy disk images (and sometimes only 1.44 disk images) with standard sector numbering. That means that you won't be able to boot 5.25 disk images, and some disk protections are not recognized.
About usage:
HxC needs a program to convert disk images to hfe files (the ones recognized by HxC), and a minimal setup (copying a preferences file to the SD card) to work.
Gotek and every other chinese emulator will work by simply copying img files to USB drive.
I have traveled across the universe and through the years to find Her.
Sometimes going all the way is just a start...
I'm selling some stuff!