Without doubt the first one, the original and the best. A truly iconic game with one of the best soundtracks in any video game it set new standards for scrolling beat 'em ups.
Hmm, it's strange. You know, thinking about it I would say Double Dragon 2 is the best of the series but when I fire up MAME I always tend to play the original. I guess it was my favourite arcade as a youth so I go back to it for that reason.
I could never get on with the original for some reason Instead I'd pick Rage of the Dragons, a nice fun tag team fighter that I enjoyed playing quite a bit of for my Neo Geo MVS.