1) Age of Worms. Completed every module, taking us nearly a year of weekly sessions. Best campaign ending in over 20 years of DM'ing, and some of the most memorable set-pieces. "Prince of Redhand" (the "low-combat" social interaction module) alone was spectacular, but there were many other highlights (the arena, Filge, Dragotha, dragons vs giants...).
2) Rise of the Runelords. Just started Module 4, and we're having a blast. Not quite as epic as Age of Worms, but dripping with atmosphere and imagination.
3) Savage Tide. I thought this would be cooler than it was. I was really psyched about it, and the first 4-5 modules were amazing. And then two things happened: 1) the difficulty level reached a point where my players were losing characters nearly every session; and 2) they started realizing that it was all demons, all the time. Age of Worms kinda fell into that syndrome with undead and... uhh... worms. But at least they broke 'em up with some other stuff. Savage Tide does get awfully demon-y in it's second half. And surprisingly, my players got tired of it.
4) Shackled City. ...and even this path wasn't *bad*. It just wasn't up to the level of the other three. I'd still replay Shackled City in a heartbeat, over some of the alternatives (World's Largest Dungeon, Rappan Athuk, Return to the ToEE, any of the Expedition to... supermodules).
Not included above, but worth mentioning, is Red Hand of Doom. It's not really an adventure path, but it's almost big enough to count as one. I'd rank that as #2 above, in terms of my group's enjoyment of it. Even though they went through three complete parties of character deaths...