If you must run Windows 10... #

Heaven help you:

Barring divine intervention, Windows 10 Enterprise LTSC 2019 (formerly LTSB) may be the least bad option:

no Cortana
no semiannual feature upgrades
not a single tile on the default Start menu
telemetry can be set to 0 (the lowest level)
no "Show suggestions occasionally in Start"
despite some misbelief, WSL and Hyper-V work just fine
receives security and stability updates for ~~ten~~ years (unlike Windows 10 Home & Pro versions, which reach "end of service" after just 18 months from their initial release) UPDATE: Microsoft is cutting Windows 10 Enterprise LTSC support from ten years to five
no Metro/Windows 8-style/Modern/Windows Store/Universal/Windows apps like Store, Edge, Calendar, Camera, News, Weather, etc.
no zombie games like Candy Crush Soda Saga, Bubble Witch 3 Saga, March of Empires, etc. that refuse to die (part of Microsoft's plan for "post-license monetization opportunities beyond initial license revenues" much like the recent Mail app debacle)

However, be aware that LTSC requires a volume license (VL) agreement, making onerous audits potentially more probable:

(despite Microsoft's own apparent need for a thorough inspection:

While five licenses are required to enter into a new VL agreement, they need not all be for LTSC. According to a rep I spoke with at a Microsoft Partner, this combination would work as an upgrade from a Windows OEM license (i.e., it would allow a user who bought a PC with Windows 10 preinstalled to run Windows 10 LTSC instead):

1x Windows 10 Enterprise LTSC 2019 Single Upgrade Open Business (KW4-00190)
4x Microsoft Identity Manager - 1 User CAL - Open Business (NK7-00031)

Here are links and prices from three well-known Microsoft Partners, though you should definitely confirm details with a rep before purchasing:

Provantage:
- Windows Enterprise LTSC 2019 Single Upgrade Open License Program NL $269.04
- Microsoft Identity Manager User CAL SA Open Business $7.81
Connection:
- Microsoft Corp. MOB Windows Ent LTSC 2019 Upgrade License Only $294.93
- Microsoft Corp. Open License Identity Manager User CAL Software Assurance Only $8.68
CDW:
- Windows 10 Enterprise LTSC 2019 - upgrade license $295.99
- Microsoft Identity Manager - software assurance - 1 user CAL $9.99

For those hapless souls stuck on Windows 10 Home or Pro, see:

Optimize-Offline, a Windows Image (WIM) optimization script which aims for "the removal of unnecessary bloat, privacy and security enhancements, cleaner aesthetics, increased performance and a significantly better user experience."
O&O ShutUp10, a portable app for tweaking Windows 10 security, privacy, and many other settings.