Sonic Research Turbo-Tuner ST-300

I also like that Walter Becker of Steely Dan used it, and Steely Dan is one of, if not at the very top of my list of music heroes. Yes, it is pricey, but I got sick of my snark tuner's battery dying on me. If you decide to buy this unit (and of course... USE it), your tuning will always be spot-on.

Obviously, this guy gets my endorsement.

Whereas there are styles of music where you can get away with a little slop as it pertains to tuning, when you play music that uses more articulate chords, your gain level and tuning becomes absolutely critical. The topic here is tuning, so let's focus on that!

The ST-300 tuner is as exact as I've ever used. I like it because:

It's a super accurate strobe-tuner!
It sits directly at the beginning of my signal chain and I can turn it on / off whenever I need it via a footswitch.
When I turn it on, its output is disabled as I tune. In addition, its display is easy to read - large, and not blindingly bright like a lot of today's pedals. Both of these features make it great for live use.
I leave it in chromatic mode which lets me tune to any note I like. This is great for those of us who use alternate tunings... which I use fairly often.