1. Mechanical - for good physical feedback. Choose silent or clicky or bump to match your auditory and tactile feedback preference.
2. TKL - no 10-key pad for enough mouse space. Still have arrow keys and friends (home, page up/down, end, delete, etc. that can be assigned to stuff or used as-is).
3. Low profile - for shorter finger-typing throws and better wrist/finger ergonomics IME.
Honestly, the HAVIT HV-KB390L was the best IMO, and I don't think there's an equally good mechanical TKL low-profile with which to replace it that is as good in all three categories currently): https://www.prohavit.com/products/hv-kb390l-low-profile-mech...
Never considered the numpad for block comments though, that's a neat call.
Anyway, I am reminded of Xah Lee, http://xahlee.info/kbd/keyboarding.html
One of his insights is that Alt and Ctl keys on both sides of the spacebar improves ergonomics (particularly for eMacs). I still find it useful years after I stopped using eMacs. That means I favor Thinkpads.
Which reminds me that keyboard ergonomics include track pads, mice, and other pointing devices in many typical contemporary uses, e.g. if you mouse right hand, the left side of your keyboard needs to do more.
I also solved the lack of accented chars by just mapping RALT+<char> into it's accented version. Took me about a week to learn but now it comes naturally and I'll never go back to my local keyboard layout.
Of course trying to find a laptop locally with US ANSI layout is a pain...
I think the combo of ortholinear, concave shape and multiple buttons for the thumb skyrocket my comfort level. Also, splitted to keep my hands at shoulder width is something I really appreciate.
But I guess that the key feature (that many keyboards share) is the ability to create multiple layouts. A dedicated numpad withouth moving the hand, a symbols layer, improve a lot the typing experience. I go with classic QWERTY with some minor tweaks, and is good enough.
Logitech K120 is ok as a dirt cheap option (you can get several, and not care if you spill a drink on them).
The standard US layout is of course ubiquitous, anything else is kind of annoying to program on, e.g. even UK keyboards have ", @, |, etc in the wrong place.