Linux

Its the most simple handheld gaming PC by far. As with all unfamiliar systems, there is a learning curve that exists, and person to person the difficulties in this will vary. But at a base level, out of the box, the SteamDeck is almost as simple to grasp as something like a Switch (or any other console). If you just want to game, and just want it to work, SteamDeck is your best choice.

Full disclosure though, the deeper you choose to dig into advanced use, the more complicated this question becomes. If you are more familiar with Windows, then using the desktop on a Windows handheld will naturally make more sense at first. But if you are comfortable with Linux (or put in the time to learn), the SteamDeck is far easier to use fully handled than it's competition thanks to easy to remap on the fly controls and the track pads. But again, this second "advanced usage" point is moot if you just want to buy games from Steam and have them work out of the box.