The market for clip-on tuners has grown rather crowded ever since Intellitouch made a big splash back in 2001 (we do, of course, still sell Intellitouch). With new brands upping the ante in both performance and affordability, it seems like there's always a new clip-on to beat the rest.
The undisputed king of the hill these days has to be Snark. The tuners grab notes quickly and accurately, the display is uniquely easy to use, and the price is very appealing. If you're in the market for a clip-on, you'd have a hard time doing better than this (or one of Snark's other models). Plus, if you're looking for a pocket tuner, Snark's other tuners (they call them "console tuners") are also worth a serious look.