Its time for me to show off my Zelda nerdiness. Although Ganondorf is always the same character, originally because of his demonic Ganon form then because of Magical Reserection, the Link & Zelda in the Zelda series aren't always the same Link & Zelda, they're usually only the same for a couple of games. However, it is said that the Links and Zeldas are reincarnations of the original Link and Zelda of Ocarina of Time & Majora's Mask. This is because of the Goddesses, Nayru, Din, & Farore's, divine interference with Ganondorf, for whenever he should arise, there will be a Link and a Zelda to oppose his Dark Designs.
I've paired together and color coded the games where they are the same Link as previously used Link.
Legend of Zelda
Zelda II: The Adventure of Link (However, the Zelda in this was a different Zelda, one put into a magically induced sleep even before the events of OoT and also the reason why every princess is named Zelda)
A Link to the Past
Link's Awakening
Ocarina of Time
Majora's Mask
Oracle of Seasons
Oracle of Ages
Four Swords
Four Swords Adventure
Wind Waker
Phantom Hourglass
That leaves these games as the only current Zelda games featuring a Link that wasn't yet used for another game: Minish Cap & Twilight Princess, although chances are the next Wii Zelda will use the TP Link again.
Note: The Link used in the Super Smash Bros. series is suppose to be an embodiment of every Link, despite using the appearance of the most recently used Adult Link. Young Link is also used like this in Super Smash Bros. Melee, Toon Link, however, is meant to be the same Link seen in Wind Waker and Phantom Hourglass due to the trophy description for him, even though he was a Young Link replacement.