This is nearly an impossible question but here goes.
Don't ask me to rank these three.
I think they are the best that MB has ever produced, for different reasons:
: Still holds its own versus modern powerplants.
: I'd say this is the motor that created the "everlasting Diesel" legend. Unfortunately this is also the motor that created the "these have been known to go 500K miles with no problem!!" statements from Ebay shyster after Ebay shyster.
: This motor makes me wonder why anyone would have opted for the straight-sixes in the US E-classes from 1992-1997. Mine is a 1992 with 133K. It's as smooth as brand new, and with 270HP on tap, it's light years away frrom the M103. There is essentially no mileage penalty - if I drive conservatively I can get 20.5 MPG in city driving and 25 freeway. Will I trade 1 MPG for an extra 90 HP over the M103? You bet.
I also have never heard of any inherent design flaws with the M119 besides the lifter oiler tubes which is an easy fix. Even the wiring harness (which went bad on my car) costs less than the harness on the 6 cyl.
...of course the reason to opt for the 300E over the 400E when new was the steep price premium of the 400E when new... I'm more than happy to let someone else pay that depreciation.
My rankings are no doubt biased as I currently own two of the three. Hopefully in the next year I can add an M100 to my stable.
Matt in Atlanta
Black-on-black daily driver
299K and laid up right now
1992 400E, smoke silver-on-mushroom, sold at 168K