Shannon and Sayid. They may not have lasted as long as some of the other couples because Shannon died early on (boooo!) but आप could always tell that somehow they would end up back together.
Rose & Bernard are the best couple on the show. The thing is, people have प्यार all wrong. What Sawyer and Kate had was unhealthy, though it be romantic and exciting in the moment. What Kate and Jack had was distrust and not believing in one another no matter what was surrounding them (not to mention the fact that Kate is pretty much the most selfish person on the show). There was young love, passionate love, lots of lust, and much betrayal and lying in every relationship on LOST, save one:
Bernard & Rose. Their प्यार transcended so many things: Race, religion, distance, time, age. They were soul mates and that kind of प्यार isn't easy to find.