If you're willing to pay $10-12 for an iOS app, MD Ballistics is my favorite for using different reticles, and it's ballistics engine seems to be spot on. It also has the option to use Density Altitude for calculations, which I really like. MD Ballistics also can use multiple b.c. values across different velocity ranges (like Sierra and Hornady publish for their bullets), so that's an interesting step up from most others.
iStrelok has a lot of reticles as well, but I don't like the user interface very much.
iSnipe has the best user interface by far IMO, and has Litz b.c. values in the library, but the reticle options are limited to some basic mil-dot configurations and a few Nightforce reticles. I have used this app more than any others but am starting to use MD Ballistics more too.
I have Ballistic AE (Applied Ballistics) as well, but rarely use it because the user interface is so bad.