LogTen Pro, for sure.
Without a doubt. And as you noticed, there are a few versions available.
LogTen Pro Universal is the version that runs on all iOS devices (so the single app purchase will allow you to run on iPad and iPhone both). It's $79.99. An
iPad-only version is $59.99 and an
iPhone-only version is $39.99.
LogTen Pro for Mac runs (you guessed it) on a Mac computer. It's $99.99.
If you have both a Mac and the iOS device(s), you can sync data across them all which is quite nice. iCloud support is also available.
The apps also allow digital signatures (sign name with finger), photo record keeping etc. My CFI's have all signed my LogTen logbook on my iPad and I have digital copies of all the paper records (endorsements, medical card, aircraft registration and airwortiness, etc.) in LogTen as well, photographed/scanned with the camera on the iPad/iPhone. Of course, some of those I also carry as physical paper (registration, airworthiness, medical).
Other bonuses include integration with ForeFlight on the iOS device (send the flight plan to LogTen Pro to record it there rather than manually typing it in) and for people with substantial paper log books to convert, a partnership with convertmylogbook.com so you don't have to do it yourself if you don't want to.
Reporting and log book page printing in a variety of forms is great. And the app is geared for everyone from student to instructor to ATP.
Coradine Aviation makes the software. Great customer service and some terrific people there.
Info specific to
flight instructors use of LogTen Pro is here. And there's a form there that CFI's can use to enter to try to win one of the two free copies of LogTen Pro Universal they give away each month.
Oh, and I also wanted to be sure to mention: Teaching your 13 year old son to fly... Priceless. And truly awesome!