I've re-watched FLCL about 10-12 times, and it's genius every time. I've also re-watched Rurouni Kenshin: Tsuiokuhen around the same amount of times (I used to watch it religiously when I was younger, and I'm currently re-watching it)
I guess my most re-watched series would b either Cowboy Bebop, Outlaw Star, or Trigun. I don't really re-watch stuff unless I'd seen it on television and then purchased the DVDs later on down the road. It's not as fun and captivating when you already know what's going to happen.
Definitely Ghost in the Shell: SAC. Just one of those shows that no matter how many times I watch it, I never get sick of it. And I usually pick up something new that I never caught before. I think I've completely watched it through about 5 or 6 times.
I also like re-watching Martian Successor Nadesico and Bubblegum Crisis(OVA and 2040).
xxxHOLiC: A Midsummer Night's Dream = 6 times.
I love this movie - awesome art, awesome music, creepy and mysterious plot, and Yuuko! I have it on DVD, which is partly why I've seen it so often. I've also forced it on a few friends, and watched it with my school's anime club, too ^^
Next is Millennium Actress (seen it 5 times). Beautiful movie and I never get tired of it.
Cowboy Bebop when I used to watch adult swim and midnight run. Wasn't intentionally trying to watch it but it just happened to be on and i was to lazy to change the channel, though I always tuned into the last episode and mushroom samba cause those where my favorite episodes.
Oh and 08th MS team for them same reason. closely fallowed by 0080 and 0083