A Series. In production for 49 years.

en.m.wikipedia.org/wiki/BMC_A-series_engine

In terms of how different the vehicles fitted with it could be, the Rover V8.

I'd suggest PSA's XUD diesel in various capacities. First installed in either Talbot Horizon or Peugeot 305, then 205, 309, 306, 405, 806 and 406 plus some vans. VArious Citroens including BX, Xantia, ZX, Synergie and Berlingo. Also some 200/400 series Rovers.

Then there's the Ford AJD-V6 diesel engine - used across the JLR range (apart from the F-type and I-Pace), Peugeot and Citroens and now about to power the F-150 truck.

Out of those I would have said the Kent and its derivatives was the most ubiquitous, it had a hell of a run from 1959 all the way to 2002.

I would say the Ford Kent, Pinto and Zetec engines.

Together possibly but individually no way.

The Kent engine (if you include the pre x-flow) is certainly numerous having been used from the early 60's in the 105E Anglia through the 70's, 80's and into the mid 90's in the Caterham before supplies ran dry. It was used by Ford Europe, South Africa and Australia (probably more). In recent times Ford Racing have reintroduced production of certain parts but the prices are eye watering. A new 1600 x-flow block is £1750 inc vat, a 1500 pre x-flow block is £2000 inc vat.

The Pinto ran from the early 70's to the mid 90's and I would say that since it was used in products by Ford USA in addition to the above its numbers might be higher than the x-flow.

The Zetec had a relatively short life from the early 90s to 2004 when it was replaced by the Duratec. But perhaps it should be grouped with the CVH which ran form the early 80's to the mid 90's since its basic structure is shared with that earlier engine. CVH heads have been fitted to Zetec blocks and vice versa.

Forced to say either A series or X-Flow I would say A series.