I'll never get bored of Halo CE's Campaign, especially on Legendary. The core mechanics are timeless, and better than any other Halo, so that alone makes it impossible to get old.
Reach is probably my second favorite Halo campaign, or maybe Halo 2, but it doesn't really matter because they aren't in the same league as CE. I have replayed the Reach Campaign a good amount, and enjoy most of the missions, it's well put together and paced pretty well also.
I got bored of Halo 2's Campaign pretty quick, Legendary is mostly cheap and not that fun, but it is the most fun Halo Campaign to glitch out of and screw around with the boundaries of levels.
Reach is probably my second favorite Halo campaign, or maybe Halo 2, but it doesn't really matter because they aren't in the same league as CE. I have replayed the Reach Campaign a good amount, and enjoy most of the missions, it's well put together and paced pretty well also.
I got bored of Halo 2's Campaign pretty quick, Legendary is mostly cheap and not that fun, but it is the most fun Halo Campaign to glitch out of and screw around with the boundaries of levels.