Tough call, I'm very flexible between jobs, so I tend to enjoy them all. I guess if I had to choose, I'd say I'd have to pick either PLD or SAM.
PLD was my first great love. I became the centre of the battle, the nucleus of our atom that my party-members revolved around, With good healing, popping out a Provoke here and there and taking damage like a trooper, it really gives me a sense of purpose. Any other job feels kind of detached after being so involved in the battle.
SAM is my more recent love. Want to hit twice as hard as anyone else in your party? Want to throw Weapons Skills out like "Cure" castings? Want to be constantly involved in weaving amazing Skill Chains together for maximum damage? Want to not only have a rocking Great Katana, but archery skills nearly on par with a RNG? SAM is the most frantic damage-fest job I have ever been involved in, and I have to bite my lip from squealing with excitement when I Self-SC for damage in the hundreds. ^_~