Surely the SL100/CRF230 motor rates a mention?
Ditto the Z50 donk, that's been in production for fifty years, copied by Yamaha and Suzuki as well as the Chinese. 3 speed, 4 speed, auto clutch, manual clutch, 50cc, 165cc, etc.
I'm no real fan, but the Yamaha YZF250 motor is a good 'un - same basic donk since 2001, and unquestionably the most reliable/longest lasting performance 250F motor out there.
The KX500 motor would surely also rate - same basic architecture from 1985 to 2004, super easy to ride, still Fast by modern standards.
Or maybe the 1977+ YZ125/DT175 donk. Light, compact, ultra reliable, with a good six speed box and an almost thirty year production run.