Short Answer: Korg OT-120 Best Tuner

The best electronic chromatic tuner is the Korg OT-120 Wide 8 Octave Chromatic Orchestral Tuner. The Korg OT-120 uses a physical needle for precise, responsive feedback. It can be adjusted to pick up the range of any musical instrument. It's the most versatile handheld tuner.

Handheld Chromatic Tuners

Korg CA-40 Chromatic Tuner"Pocket" tuner is not an official category, but rather our name for inexpensive electronic chromatic tuners designed for routine use by musicians. They are easy to use and generally small, though not all will fit in a pocket. They are low cost, from $10 to $150. For example, the Korg CA-40 Large Display Auto Chromatic Tuner pictured here has everything most musicians need for under $25. Higher priced units add features and detect more octaves. Most pocket tuners use the needle display or its variants; many add lights for more feedback; a few use strobes. Most low cost pocket tuners reliably detect octaves in the middle ranges. Most wind and string instruments tune within that range, but the higher and lower octaves in pianos and marimbas may need more robust models such as the Korg OT-120 described below. Inexpensive pocket tuner displays with only lights or low-resolution LCD needles may not provide enough precision for some uses. Useful features to look for are the ability to be calibrated, play tones through a built-in speaker, or add an external contact microphone such as the Korg CM-100L Clip On Contact Microphone For Tuners Advanced tuners may offer alternate temperaments or have transposition switches for instruments tuned to different keys. Special types of pocket tuners include:

Korg GA-1 Guitar Tuner detail

Guitar Tuners

Electronic guitar tuners are often simple for maximum portability and ease of use. Note that some dedicated electronic guitar tuners may not be full chromatic tuners; they may only tune the notes that guitar players need. They may have features specific to guitars and bass, such as dropped semi-tones. More advanced models have the ability to select tunings other than the EADGBE standard or chord finders. Highly rated guitar tuners include the very compact Korg GA1 Guitar and Bass Tuner; but for a few dollars more, the Korg GA-40 Large Display Guitar and Bass Tuner adds a larger display and a sound-out jack. If you want to buy an electronic tuner labeled for guitars to tune other things, make sure you read the description carefully. The tuning features of most basic guitar tuners can be replicated by full chromatic tuners.
Korg Pitchblack True Bypass Chromatic Tuner Pedal detail

Pedal Tuners

Pedal tuners are used on stage by guitarists and other stringed instrumentalists. The electronic tuner rests on the floor and is used with a "pick-up" mic wired directly the instrument; this eliminates ambient sound. The player has the option of muting or turning off amplification so that he or she can tune without disturbing the audience or other band members. Other features are geared to stage settings, such as extra-bright displays visible outdoors and "bypass" mode which takes the tuner out of the amplification circuit during normal playing. See the Korg Pitchblack True Bypass Chromatic Tuner Pedal as an example.
Korg AW-2 Clip-on Tuner on guitar neck

Clip-on Tuners

This modification of the basic pocket electronic tuner clips directly to the neck or bell of the instrument. It "hears" by using direct contact to sense the vibrations of the instrument itself. All tuners function better the closer to the source; this is about as close as one can get. This reduces the interference of ambient sounds, and like a pedal tuner can be used silently on a noisy stage. Feature sets vary, but these tuners tend to be simple to keep the size down. Most models emphasize string and guitar, and can be left attached while playing. Many wind instuments can use them, though the positioning can get tricky. The Intelli IMT500 Clip-on Chromatic Digital Tuner for Strings is a popular example; Korg's entry is the AW2G Clip-on Chromatic Guitar Tuner. Korg CM-100L Clip On Contact Microphone

If you need the accuracy of a contact mic, but want or already have a standard handheld unit, many tuner brands offer contact mic as an add-on, such as the Korg CM-100L Clip On Contact Microphone that works with tuner with an audio input jack.
Planet Waves Tru-Strobe Tuner detail

Pocket Strobe Tuner

Pocket strobe electronic tuners use a strobe display instead of a needle, yet are otherwise comparable to other pocket electronic tuners. These use lights that move a circular pattern rather than spinning discs. The Planet Waves Tru-Strobe Tuner has very solid reviews.
Korg OT-120 Orchestral Tuner detail

Advanced Compact Tuners

The features than can be packed into small tuners is amazing. Orchestral electronic tuners like the Korg OT-120 have the most features among pocket tuners under $100. The OT-120, for example, has several advantages over lesser models. The best feature of general interest is the analog needle, which can provide more accuracy than a "jumpy" LCD needle. The OT-120 can also can detect, and play, a full eight octaves. Designed for orchestra and band instruments, it has a transposition switch for quick adaptation to different tuning keys. Finally, the OT-120 is also able to tune several temperaments other than equal temperament. It will tune about anything, and even assist a piano tuner who does want a professional piano ETD. If a strobe display is preferred, consider the Peterson VS-F StroboFlip Virtual Strobe Tuner which has even more features than the OT-120, including some special ones for guitars, albeit for $200.
Boss TU-80C Chromatic Tuner and Metronome detail

Combination Tuner and Metronome

One feature many find handy is to have a metronome built into the electronic tuner. This variation can be found among the many styles of pocket tuners. The Korg TM-40 Large Display Digital Tuner and Metronome is an excellent tuner as well as metronome.