Portal 1 and 2. The spin off Aperture Desk Job is also an introductory game for the Steam Deck, explaining all the controls.

Apart from that there should be a top games played on Steam Deck somewhere around here.

When you start playing shooters on the Deck try to get comfortable with gyro aiming and maybe flick stick. Many games don’t have aim assist on the PC, so aiming with a stick is harder than on consoles.