What are your favorite Photoshop shortcuts or hidden features that make your workflow more efficient

When recording an action, it is helpful to know the shortcuts Alt/Opt + . (period) and Alt/Opt + , (comma) to select the topmost and bottommost layers in the Layers panel. Clicking on a layer will hard-code the layer's name into the action step.

[Alt + Right-click + Drag Mouse]   Left <>Right Modifies Brush Size and  up<>down modifies Softness.

It depends on your typical workflows.  If you find a particular function is annoying or frustrating, then see if it has a shortcut.  If it does not have one, then give it a shortcut.  I hate the time it takes using right click menus. They have got longer over the years, and while they have made them contextual (contents are omitted if irelivant to the layer) it can still take a moment or two to look down the list to find the function you are looking for.

This is the layers panel right click menu, and what you see if the layer you clicked on is a Smart Object. The ones I use many times a day are Convert to Smart Object and New Smart Object via Copy.  I use Rasterize Layer a fair bit as well, so I have actions that are triggered by Function keys to save me having to use the menu.  I have actually just seen one I hadn't realised was there!  My magenta highlight Convert to Layers.  Up until this moment I haver done that by dragging the layers back from the SO edit window. I don't use it a lot, but I'll be making another action (Ctrl F12) after uploading this post.

Another big time saver is removing shortcuts from tools you seldom use.  If I hit the B key, I will always get the Brush tool, and never the Pencil, Color Replacement or Mixer Brush.  When I hit G I will always get the Gradient tool, and never the Paint Bucket.  I have repurposed the Y shortcut from a function I never use (I think it was the Art History Brush)

I use Function keys to set and reset my workspaces, I have unchecked using the Shift key to change tools.

If you have the screen space, expand your Toolbar.  I think we all use all of the Shape tools, and there are six of them, so I use the mouse or stylus to select them. Similarly, group all of the minor panel to the right of screen panels.  Click one to twirl it open, click again to twirl it closed.

Alt+Shift+Ctrl+E for Stamp Visible. Not accessible though the menu; keyboard shortcut only.

Are you doing research for a test or an article or something? 

Your two posts so far seem a bit peculiar to me.