If you are talking about performance this poll is unnecessary- it is the 340 hands down.
The 340 was one of the best engines of the 1960s and 1970s. It had high-flow heads, big ports, a two-level intake manifold, and a six-barrel option (three two-barrel carbs). When it was introduced in 1967, it quickly became known as the "giant killer" by its success on the track against big block cars. The high-winding engine was rated at 275 hp but the NHRA factored it up to 325 hp (Plymouth "cheated" by taking the hp readings at 5,000 instead of 6,000 rpm).
It was more than just 22 cubic inches bigger than the standard-performance 318. It had:
A dual timing chain and windage tray.
Huge 2.02 inch intake valves and 1.60 inch exhaust valves.
A high-rise dual plane intake.
An 850 cfm carburetor (from 1971 to 1973)
A forged steel crank.
High-performance heads.
A revised oil pump with a 90 degree adaptor.
A special cam.