Batman and Catwoman
I was on the verge of declaring Gotham journalist Vicki Vale as Batman's longest running romance -- other than Robin, obviously.
But while Vicki did get perfectly cast (Kim Basinger, and picking a blonde Bond girl is always a right move) a buddy pointed out that Batman only really got his groove on with Selina Kyle, Catwoman.
The two have a rollicking love-hate relationship over the years, a very sexy dynamic indeed, and while Michelle Pfeiffer made Catwoman even special-er than the comics, nobody could purr like Julie Newmar from the old TV show. [Can we please not mention Halle Berry? Please?]