That's a really hard one to judge. I have seen 1/2 million mile engines with the Ford 300 6, the Chevy 250 6 & the Mopar 225 slant 6. If I had to choose between them, the 225 slant six was almost impossible to kill. When I was a teen, I had one that I ran with changing the oil for over 30,000 miles! They weren't the most powerful, but they were tuff. As far as the 318 engine, you either got a good one or a flat out turd, no in between. Had one in a 73 Dodge Challenger that had 200,000 miles on it & got 27 mpg & would burn the tires down to the cords. If you want a performer though, the Chevy 327 is the one! I don't think GM has produced a better engine for performance period. Big block Chevys are too easy to blow up, they aren't very forgiving if overreved or overworked. With the big blocks, the Mopar 440 & Ford 390 were awesome!
This is based on gas engines only.
On the big trucks, The 425 CAT rules!