The 8BitDo Ultimate controller 2.4ghz version is the best for the steam deck. They released a cheaper version if you don't need/want the dock. RetroGameCorps has a video comparing all the different versions of this controller since it is a bit confusing.
If you prefer 2D games and primarily use the d-pad then the 8BitDo Pro2 controller is best for this. Note this is only a Bluetooth controller and for fast pace games you will notice some input lag on the steamdeck. I bought a 15ft usb cable to just plug it in and play on the couch. I haven't tried 8BitDo's wireless dingle with it to know if this removes the input lag. I might try that this evening to find out.
Tunic was such a great game. Good difficulty and tons of exploration. Some of the shortcuts seem to be invisible until you come out the other direction then it is obvious.