Any amp works fine.
Acoustic is best if you want a clean acoustic sound.
Non-acoustic amps (electric guitar amps?) really depends on the amp, as they're not specifically designed for acoustic instruments.
They will often colour the sound of the instrument, so it sounds more like an electric guitar.
This can range from sounding like a cruddy broken old radio, to a good quality electro-sound with effects, depending on the quality of the amp.
Some electric guitar amps that have good rep for acoustic ukes are Rolands and Vox.
I use my Bass guitar amp (a VOX Pathfinder 10W Bass) for my acoustic instruments, because bass instruments are "similar" to acoustic amps.
This is convenient for me, since I also play bass guitar. Two birds, one stone.
I also have a Vox AC4TV, which is a tube amp for electric guitars. Plugging acoustic instruments into it does tend to give it a bluesy, crunchy tone that works in some genres.
I also have a Roland Microcube, which is sorta my beater "travel" amp. Plenty of options for uke there, ranging from a reasonably clean acoustic-tone to more electric sounds + effects, depending on how I have it set.