I'd personally go for Track & Field and Karate Champ, despite my not knowing which of these four are the hardest to come by "Down Under".
My arcade library will eventually encompass games like those two that are at least a *tad* obscure{at least in comparision to Donkey Kong and/or Galaga}.
Ultimately, pricing would make a difference if I was in your shoes, too{unless they're all priced within the same range}.
I recently had the opportunity to play Track & Field in its original arcade glory{after years of NES-only play}, and it was both entertaining and challenging{thanks, Jeff F}!
Double Dragon is a bit of a wild-card, as you can't really ever go wrong with a DD title{I hope to someday own the Neo Geo/film version...but I need a MVS first}.
Best of luck to you in whichever games you end up buying, Jardine!