The software options for iOS are a bit limited.
I went with the LELink Bluetooth Low Energy BLE OBD-II OBD2 Car Diagnostic Tool For iPhone/iPod/iPad and chose the OBD Fusion app for software.
I chose that OBD reader because it is low profile and I have long legs and wanted to make sure that I wouldn't hit it while driving.
of Volt PIDs is very useful, but there are some minor formatting changes required for the equations to work with the OBD Fusion app (e.g., change lowercase letters to uppercase).
I'm using this combo to give detailed information on ICE battery regeneration so I can see when the car is trying to replace charge above background levels. Using the iOS split screen, I can have Waze running on one side of the iPad screen and OBD Fusion on the other. It's pretty slick.