I hate to be a contrarian, but I think the Desmond/Penny content can get a bit overwrought.
I still like it, and "The Constant" is great (I gave it a 10), but I don't agree with the semi-popular belief that Desmond and Penny are the soul of Lost. I'm not sure they quite pulled off what they were ostensibly going for — an epic romance that conquers space and time.
Desmond and Penny sometimes read to me as playing structural roles in the show. (At worst, Penny herself can resemble a macguffin.)
But Jack and Kate read as flesh and blood characters with explosive chemistry and an existential connection that seems to transcend earthly concepts of friendship or romance. In fact, when they are together and engaged off-island, rather than an end-point it feels like just one hat they're trying on in their eternal journey. In some moments they even achieve this intense look of recognition as if they remember each other from a past life. (For example, 14:40 in "I Do.")
Perhaps it has something to do with Matthew Fox and Evangeline Lilly being the best actors on the show. I don't know if that's controversial, but I truly believe it.
Watch "Something Nice Back Home" and "I Do" and "316" and see if you agree.