The original, hands down. It was the only series that had to fight for every season to stay on the air, and even got canceled once! It's also got all the humans differentiated by nationality. The others, I feel, clump the humans into one group and make characters different though species, instead of nationality.
However, I am biased because it was the first series I'd ever seen, and I've never even seen DS9 or Enterprise. I even went to Comicon this year dressed as a redshirt!