If you're asking about the AMP (not the DAC/Amp combo), I really like the sound of RSA amps. I own a Predator (it's a DAC/Amp), but Ray also offers amps only: The Hornet (http://www.raysamuelsaudio.com/products/hornet), The Tomahawk http://www.raysamuelsaudio.com/products/tomahawk and a few more). They are noticeably on the "dark" and "warm" side of things though, so if this is not your cup of tea - look elswhere. To my taste, RSAs easily better similar iBasso products, but everyone's taste is different.
Another great option to consider is HeadAmp Pico family. http://www.headamp.com/pico/pico_amp/index.htm I haven't owned one, but quickly auditioned once. Great sound, lighter and more open than RSA, but as a consequence a little less "substantial". I heard only the original Pico, they offer the Pico Power now, which is reported to sound noticeably better.
With no offence ment, I am personally not a great fan of FiiO sound. I admit they make some really great products for the price, but other amps simply sound more to my liking.
I hope my suggestions fit your definition of "mini" -)