There are quite few quality manufacturers of engines. Rivarossi, Broadway Limited, Atlas, Kato, Life-Like's Proto 1000 and 2000, Bachman's Spectrum series. I would even include Athearn Blue Box. They are the best entry level engine and have good price points. I have some that are over 40 years old and still running. I am not really familiar with the European manufacturers, so I cannot venture a critique there.
One's I steer clear of are TYCO, Model Power(the little starter engines found in the starter sets), and the entry level Bachman's. I don't care for the talgo wheelset/coupler configuration that has the coupler attached to the wheelset. Such a configuration does alleviate some derailments, but just doesn't look authentic. And NO I'm mot a rivet counter, but do like some sort of realism. I model, not play with toys. A bit of a difference.
One can usually tell the difference in quality just by holding the different engines, looking at the detail, and chassis.