While Apple does not appear to publish the maximum charging specifications for iPad, newer models are reportedly able to charge at up to ~28W. As such, a 30W USB PD (Power Delivery) Power Adapter will permit the fastest possible charging rate.
You should note that the maximum charging rate will only be available up to approximately 80% of the iPad's maximum battery capacity - as the charging rate is intentionally reduced to protect the battery during the final 20%. The charge rate is heavily dependent upon temperature; if too warm or too cold, the charging rate will be reduced.
The charging process itself creates heat - as does actively using the iPad while charging. High screen brightness and/or CPU/GPU intensive processes adding to internal heat generation.
Higher-output Power Adapters provide additional benefit when being used with other USB devices via a USB hub. Higher-output Power Adapters can provide a greater “power budget” - allowing other devices to receive power while maintaining the iPad’s maximum 20-30W charging rate. For example, a USB multiport hub may typically require 10W power; connected USB devices (external storage, keyboard, mouse etc) also draw additional power from the source. If the connected load exceeds the available supply, the iPad may not charge - or may charge at a substantially slower rate. In extremis, both the iPad and Power Adapter will share the connected load - and the iPad may actually discharge its battery to make-up the shortfall in available power from a connected Power Adapter.
You should note that while Apple devices generally support USB PD, they do not support other charging standards - such as QC (Quick Charge).
