I use an alloy roadbike or steel roadbike fixie in the sticks...
but, when I commuted between Paddington, SW8, EC4 and SW1, I had an MTB street-warrior road rocket that was honed for the heat of city tarmac, as follows:
Ancient Trek 4500 (?) hardtail frame.
Forks locked out.
Continental SportContact near-slick tyres at 80psi.
Long stem and shortened straight bars with bar-end cowhorns.
Tighter- higher-geared rear block.
Arse-slicing road saddle.
Eggbeaters.
Ridden with verve in the Big Smoke, it gave me the impression it was unstoppable (in a good way).
Kerbs and potholes were never a problem, narrow gaps were gettable and no-one seemed interested in stealing it or any of its QR parts.
City commuting on ratbike, road-converted hardtails is the way. Others disagree and are wrong.