What charger should I use?

Ideally, you would chose a 20-30W USB-C Power Adapter that supports USB PD (Power Delivery). This will allow your iPad to charge at (or near) it's maximum supported charging rate. You can use a more powerful USB-C Power Adapter if you choose to do so, but your iPad will see little (if any) benefit.

If you are using a USB Hub and additional accessories with your iPad, a 30W Power Adapter (or greater) will provide additional capacity to run the attached accessories without impacting the maximum charge rate for the iPad.

All Apple USB-C Power Adapters support USB PD - as do many third-party Power Adapters. Be aware that Apple devices do not support proprietary charging protocols such as QC (Quick Charge).

iPad (A16) can be fast charged with USB-C PD 20 watt and higher power supplies.

Wattages in excess of 20 watts won’t charge faster, but also won’t hurt the iPad (A16).

There is one other variable which has not been mentioned above. Try using a different charging cable from the power adapter, because most USB-C cables have some degree of signalling built into them to show capability. Either a defective cable or uncertified third party accessories can block USB-PD and pull the charging rate down to the lowest default, often the original 5 Watt standard from USB 1.0.