I have used just about everything, the kestrel 5700 elite with link and applied ballistics is by far the best right now. I use in conjunction with the kestrel ballistics app it let's you back up your guns you have built, and let's you access all of Bryan Litz 780 custom drag curves for free, but it is 1000 dollars. I have also used the kestrel 4500 with the HORUS atrag ballistic solver which for most people shooting to 1000m is more than adequate.( If you can pick one up cheap, do so). However OP is asking about apps, the applied ballistics app (36.99) is nice, but not as intuitive as the kestrel and relies on atmospherics from your phone, does not have the ability to tell you wind speed, and Although you have access to Litz's custom drag curves, you have to pay for each one separately (3.99 each) not a big deal as most people only a few and they are far superior to the G1 and G7 drag models. With this app you can true your MV(cal mv) in transsonic, as well as record MV in different temps ( mv temp table) and last but not least the Drag scale factor ( dsf) allowing you to gather data points into your bullets subsonic trajectory. My next choice is strelok pro ( 16.99) does most of what AB does, trueing, atmospherics from your phone, but is a little more user friendly IMO, you can true your MV or BC into transsonic (cal mv), but there is no DSF, MV temp tables, and you don't have access to the custom drag curves, for the price this is a solid app.