Depends on what your needs are.
My favorite at the moment is an iPod Touch running software called Proloquo2go. It's slower than typing with ten fingers, but it's more portable and easier than having this giant thing attached to my wheelchair obstructing my view.
There's different possible needs for:
* Input techniques (keyboard, touchscreen, capacitative touchscreen, large keys, small keys, switches, head tracking, eye tracking, morse code, etc.)
* Size, portability, ability to mount to a wheelchair, etc.
* Color (some come in truly blinding shades of green or yellow)
* Picture symbols, text, large pictures where you can click on different parts of them, etc., or some combination
* Need for quick-press buttons with common things to say on them
* Auditory cuing, visual cuing, scanning, etc.
* Durability
* Whether or not you have trouble switching pages (if you have a dynamic display device and need lots of pages)
* Whether the device crashes all the time or not
* Clarity of the voice, and also whether it's overloading to listen to
And many other things. It's very hard to recommend what kind of device you need without knowing those things and more.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams