I use Peterson Stomp Classic Strobotuners. No longer sold new but can be found on ebay or Reverb and are still fully supported by Peterson. They are very accurate for intonation, are very stable, can be used on a pedal board, have a built-in direct box for studio work, have sweetened guitar tunings, various drop tunings, built like a tank, can be used to chock a tire if you get a flat on the highway, or for personal self-defense. Peterson has a wealth of information on their website regarding various aspects of tuning various instruments, and instructions on intonation if you're interested, they're a good good read. If that's too much for you, they have a newer, smaller model, won't stand up to a bullet like the Stomp Classic, but still the same high accuracy as the Stomp Classic. As mentioned above, they also have the app version, which is very accurate, and they sell an input adapter cable to use with your phone, but there have been some complaints about the durability of that cable, YMMV. The app have various options and and price points, so you can get the basic tuner and add options to it later if want them. Personally, if I wanted a phone app tuner, the one I'd get would be the Peterson.