Vibrant Deck is nice because it bumps up the colors a bit and makes it seem almost OLED, but I think the actual best is Proton DB Badges. This plugin tells you if a game is compatible (and how compatible) with the steam deck. Steam official playability is notoriously conservative, plus Proton DB can tell you what might not work in a game.

Shameless plug but I just re-released my Volume Mixer plugin to the store. It allows you to control the volume of games, programs, and connected Bluetooth sources like phones. I use it all the time since I usually have a podcast or stream going on my phone while playing a game

Mine is vibrantdeck. It improved the image quality quite significantly. I think the option is now built in, but I don’t bother.