Folding bike:

Park MT-1 strapped under the seat
(if I get a flat, I'll walk or catch transit)

Non-folding bike:

Qwik-stick (tire lever)
Small allen-key multi-tool (One23 MT4 or Lezyne SV5 )
Small pliers/knife multi-tool (Mini Bear Jaws)
Spare tube
Co2 Cartridge + inflator
15mm wrench

On long rides, I add a patch kit, extra Co2 cardridge & a presta/schrader adaptor