Apart from the most reliable app for polar alignment (with or without a polar scope), PS Align pro has become the astronomer's swiss-army knife with all the tools added over the years. Remember, all features & updates are free forever, the only in-app purchases are donations (tip jar).

Polar Scope Align will calculate the position of Polaris or σ Octantis in your Polar Scope reticle for your location (using your phone's GPS or manually), allowing a quick and accurate polar alignment. It is one of the few programs that are accurate in lower latitudes by correcting for atmospheric refraction (so expect results to agree only with precise software and not most simplistic "polar align" apps - see the "Accuracy" section in help for details). In the settings you can select the polar scope reticle that matches your own, from a vast collection:
iOptron old & new, Astro-Physics RAPAS & PASILL, various versions of Takahashi EM-10/11/200/400/500, NJP, PM-1, P2-Z, the latest Orion / Skywatcher, the Classic Skywatcher / Orion / Celestron / Meade / EXOS2 / Bresser, the Telrad, Rigel QuikFinder, a couple for Astrotrac / Losmandy / Kenko / Avalon, the Vixen PF-L/Atlux/SX/SXD/SXP/AXD/GP/GP-DX/Polarie Polar Axis Scopes, a Konus and several Polar Finders: Tuthill, Explore Scientific 8x50, Meade 9x60, Vixen Polaris, Celestron 7x50, Carton Optical. PoleMaster help view for South Hemisphere users.

The Pro Watch version has all the features of Polar Scope Align Pro, with the addition of an Apple Watch app featuring:
-Graphic & Text display of polar alignment.
-Weather view.
-Bubble level.
-Day alignment & push/hop-to assist.
-Moon/Sun rise/set/twilight/phase.

Feature list:

-Accurate calculation of Polaris or Sigma Oct for any date and location, including effect of atmospheric refraction (and annual aberration, nutation). Most other apps do not perform precision calculations, so please only compare app readings to accurate sources (USNO etc).
-Use device time/date/location, or enter your own, or pick from online & offline world databases via the Location Manager.
-Approximate daytime / no polarscope polar alignment tool. Great for visual use, solar imaging (inc. eclipses) and even limited short focal length astrophotography. Now with sun shadow calibration for more accuracy!
-Laser / Hop Align for nighttime alignment using a calibration star for obstructed or not visible polar star, or for improving laser align.
-Full app in red-color (or alternative green, white) night mode - tools switchable to daytime view.
-Deep Space Object (DSO) Database with 22000+ objects from several catalogs (Messier, Caldwell, NGC, IC, Herschell 400, Sharpless 2, Barnard), along with stars (Yale Bright Star) & planets. Search, apply filters, sorting, download DSS color images for all items, or get help finding them with your non-goto telescope with the Push-To function.
-Connect to and control WiFi-enabled iOptron mounts.
-Over 30 calculators useful for observing and astrophotography, from eyepiece magnification and field of view, to imaging res and reducers.
-World astro-weather report based on multiple sources.
-Solar Eclipse Calculator/Simulator.
-Water vapor satellite images.
-List of visible ISS and bright satellite passes.
-Moon phase & lunar Calendar with solar/lunar rise/set and twilight times.
-Wizard to calculate your reticle centering offset and correct or compensate for it.
-Zoomable reticle view and customized routines & tools: Alt-az alignment error display for mounts that calculate it after alignment (e.g. iOptron iEQ45, CEM60), polar alignment when your scope is not in zero position (for mounts like iOptron CEM on which you can always access the polar scope), remember zero position on Celestron / Skywatcher / Orion etc reticles.
-Bubble level (using either gyro or accelerometer) and Compass heading, as well as magnetic declination reading.
-GPS accuracy & elevation data.
-Red/White/LED light.
-Dimming slider.
-iOptron, NexStar, Meade LX200 Star Lists.