After using Winton 201-A engines in some 1930s locomotives, EMD switched to the 567 in 1939. They used those until 1965, when they switched their whole line to the 645 engine. Starting with the SD60, they switched to the 710 engine, which they still use today. The main exception to this progression are the true 6000-hp SD90s which use the H engine.
GE used the FDL engine in most power from the U25B on. The main exception were the true 6000-hp AC6000s using the HDL engine, until the ES-series introduced the Evolution engine.
16-cylinder engines are the most common for road freight engines. At any given time, a manufacturer's "standard" model will usually have 16 cylinders. However, divergence from standard is achieved in different ways. In EMD's 1965 line, the GP/SD40 had 3000 hp from 16 cylinders. The SD45 had 3600 hp from 20 cylinders, while the GP/SD38 had 2000 hp from a non-turbocharged 16-cylinder engine. GE's U30B/C also had 3000 hp from 16 cylinders. However, the U33B/C and U36B/C came later than the SD45 (though well before EMD put that much power in a 4-axle unit). They also had 16 cylinders, taking advantage of technological advancement. To get lower power for the U23B/C, GE went to 12 cylinders. Since it didn't build road diesels in the 50s and used fewer cylinders rather than normal aspiration to get lower power, I don't believe GE has ever built a non-turbocharged road engine.
Most EMD switchers used 12 cylinders; some had 8 or even 6. E-units had two 12-cylinder engines.