I would have to say...
6V92TA,
John Jeere 6081h,
what about you?
My favorite engine is the 6v92TA! Hook that up to an Allison HT series transmission (preferably the HT-748) and I'm in completely bliss ^_^!
Heh. Series 71 and 92 for me. Especially the 8vs.
Classic's:
8v71
6V92TA
L10E
M11E
Diesel:
ISM
ISL
Series 60
CNG:
Cummins Westport ISL G
Cummins C Gas PLus
John Deere 6081H
My favorite engine would be the late 1990s-early 2000s Caterpillar 3126s.
My favs are:
DD 6V-92TA
DD 8V-71N
Cummins C Gas Plus
Classic:
6V92TA
M11
L10
8V71
Current:
C Gas Plus
ISL
ISM
Detroit Diesel 6V92TA
Cummins L10
DD 6V92
DD series 50 and 60
John Deere (whatever WMATA's upper 2600s/2700s use)
Cummins ISL and ISM