I find that the most comfortable hotkeys are single keys. Any kind of chorded hotkey will hurt eventually.
To that end, I’ve used AHK to assign as many shortcuts as possible to the function keys contextually (they change depending on the best use for each program), but for global app and tab switching, I like Win and Alt.
AFAIK, this is only possible in AHK, but you can actually assign functions to modifiers when you press and release them without touching any other key. You just have to turn them into prefixes earlier in your script. Here’s part of my always-on AHK script:
Press the left windows key to go to the previous app, the right windows for the next app; the left alt key for the previous tab, the right alt key for the next tab. I find this super comfortable, much more so than any combination of modifier+random letter/number. And the windows and alt keys can still be used in combination with other characters as conventional modifiers, these keys are only activated when they’re pressed on their own.