I think there will not be a single popular car as many have different views on what a popular and good car is, but how about you what do you think?
WCUSA: Roamer mostly, as IRL SUVs dominate among US cities. Then Ibishu cars also would be a common sight since they seems to be old and relatively cheap ECUSA: D-Series, no doubt. Utah: D-Series, T-Series and Roamer. Italy: Cherrier and low-end ETK cars.
You can check in game engine what cars are popular on what types of maps (vehicleGroupsDefault.json file). American maps: D-Series, LeGran, Grand Marshal, T65 cargobox, Bluebuck, Moonhawk, taxi and facelift Wendover, Barstow and Roamer. Asian maps: 200BX, sport Sunburst, Covet, Pessimas, Miramar, Pigeon, Wigeon and Hopper. European maps: Piccolina, Bolide, ETKs, Vivace 110 and Tograc 160Q. Rural areas: D15 Farmhand and crew cab, 4WD Roamer, Hopper, 4-door Bluebuck, H-Series, Miramar and LeGran luxe V6 wagon. Urban areas: new Pessima, ETK 2400i, Grand Marshal taxi and police, H-Series cabster, base D15, K-Series, Covet LXi, bus, H-Series ambulance, and LeGran fire chief. Fun fact: there is also a file that allows you to add your own traffic setting for a fictional country/continent in a mod map. You have to put it in settings/editor and call it vehicleGroups.json.
This probably changed completely in 0.25 seeing how they have redone most of the traffic code from scratch
Ibishu prodigy, etk 1300 in usa especially wcusa Procyon Centauri in middle east Vivace in Italy D series, roamer in ecusa
If not talking about mods, then: -In WCUSA, probably some cheaper Ibishu cars and the Gavril Roamer. Also the Wentward DT40L. The police would drive 200BXs and some other sporty vehicles. Sometimes the BCPD Roamer. -In ECUSA, probably the D-Series, Bluebuck and other boat-y American vehicles that can take slight-to-medium off-roading. The police here would drive Pessimas and Moonhawks. -In Utah, lifted Roamers and D-Series. The police would drive sheriff Roamers. -In Derby Arenas, well, derby vehicles . The only police here would be the derby Grand Marshal. -In Italy, the FCV and cheap (diesel) ETK's, if you have more money, then the Bolide. The Polizia would probably drive either the Bolide or the Scintilla. -In ETK Driver Experience Center it would be sporty ETK vehicles (mostly) and other sporty training vehicles. The police would be the ETK K-Series and 800-Series. -In Automation Test Track, mostly Japanese vehicles. The police would (maybe) drive something like a 200BX or a Sunburst.