318 - '68- '96 28 years of faithful service and abuse in every concievable platform and use can't be a bad engine...
340 - '71 with the electronic ignition and Thermoquad intake and carb. Well tuned it will spit out almost any stock big block below 440" plus a bunch of stock 440s... excepting the Hemi of course..
440+6 - Having owned a built many, I simply love the driveability and power potential of these engines. All the power and none of the BS of the 426s.
Close tie for 4th would be the 413MW, 426MW, and 426 Hemis. Al fun and powerful but tempermental and short lived.