Can’t go wrong with a Dualsense controller. Steam Input works great with it, and it has a touchpad, tilt sensor, analog triggers, and you can even remap the ‘mute’ button. Mine has the Rise4 paddles mod, which isn’t quite like the steam deck’s remappable grip buttons, but its close enough for most games where I’d want them.
I’ve been using Lutris for this purpose, so that’s another possible method!
Ironically enough, it’s led to me playing more games on the living room television! The steam deck helped me adapt to playing with a gamepad, as opposed to mouse and keyboard.
Until they come out with a Steam Controller 2, I will say the best gamepad for steam is the Dualsense (a Dualshock 4 also works). It’s got one touchpad instead of two, but Steam lets you map the left and right half separately, which covers my primary use cases. I also installed the RISE4 remap kit, a hardware mod that adds paddles on the back of the controller which can mimic any face button. Not as good as having actual new buttons, but it does mean I can run and jump without taking my thumb off the right stick.