All are inline-six diesels unless noted otherwise.
Very Good (first choices):
International DT466/466E (best of the bunch)
5.9 Cummins 6BT/ISB
6-71 Detroit (rare, except Crowns)
6V71 Detroit (V6), mostly pushers
Cummins 855 (again, mostly in Crowns)
Good (I'd own one):
Caterpillar 3116 & 3126
Ford/New Holland 6.6 & 7.8
International 6.9/7.3 litre (V
International T444E (V
Caterpillar 3208 (V
DT408 (as good as the 466, but kind of weak on power)
DT360 (see DT408, but even weaker)
Bad, avoid unless a screaming deal:
9.0 International (V
8.2 Detroit (V
Horrible, avoid at all costs:
504/555 Cummins "triple nickel"
For gas...well, it's $3+ a gallon. Anything will do well to get 6MPG.