For some reason, Apple often chooses not to include the best charging option with its devices. This is particularly odd with 9.7” iPads, which ship with a 10W power adapter. Those haven’t been available for sale from Apple since 2013.
According to the Apple Online Store, the 12W USB Power Adapter is compatible with the iPhone 7 and 7+.
Both 4.7” and 5.5” iPhones can use up to 10W power adapters to charge faster, as measured by an ammeter and System Information.
Either way, power is drawn by the “receiving” device, so there is no risk in using an adapter that provides more power. If the charger can provide the requested wattage, it does; otherwise, the highest power output is negotiated.
If you plug into a Mac’s USB port, your Mac will deliver 2.1A (10W) to iPads and to the larger iPhones (6 and above). It will deliver 500mA to WATCH and 1A to smaller iPhones (SE and lower).
I have tested this with a 2013 iMac. It is possible that newer Macs are able to provide 2.4A to iPads.
Update 2017/03/22: iPad 5 and iPad mini 4 now ship with a 12W power adapter. According to the chosen verbiage, this may be the standard going forward.
Which power adapter should I use to charge my iPad?
The 12.9-inch iPad Pro, 9.7-inch iPad Pro, iPad (5th generation), and
iPad mini 4 come with a 12W USB Power Adapter. Some previous iPad
models came with a 10W USB Power Adapter. If you need to purchase an
additional adapter, choose a 12W adapter because it can charge all