Dude. :o You're such a gay. III is amazing. The music ROCKS. The graphics are rather bright and colorful, and finely detailed. It's also by far the easiest in the series (unless you're playing hard mode, that is).
Dunno about worst. I've played Contra and Super Contra (arcade and NES versions for both), III, 4, Hard Corps, and Shattered Soldier, and loved 'em all.