Worst ride, huh? That's a tough one... Some rides that once were favorites now rank amongst the lowest with my family due to bad updates/revisions; this is true for the Journey Into Imagination (or who's ever imagination you're journeying into these days!) Bring back Dreamfinder!
The Autopia/Tommorowland race cars is not worth the ridiculous wait to ride. We gave it another chance this past trip in August, and it reconfirmed our desire to avoid in the future.
Over in Disney Hollywood Studios, the Indiana Jones Stunt Spectacular is a bit long in the tooth these days. Although I'm a big Indy fan, this show has seen better days, and should make way for the Indiana Jones ride (like in Anaheim) or give it's space up to an extended Star Wars area.
Ironically, I love most of the "most hated" rides already mentioned. To me, it's just not a trip to WDW without paying those country bears a visit (although I would rather see them bring back the "Country Bear Vacation Hoedown" version), and Carousel of Progress is tired, but still a classic and one of the few original ties to Walt (perhaps they could flip it every once in awhile with the "Now is the Time" version?). And the Tiki Room? Well, since they finally realized what a mistake the "Under New Mgt." attempt was, it's nice to see it almost back to it's original form. Yeah, the shtick is corny by today's standards, but so are the jokes in the Jungle Cruise, the gags in Pirates, and lots of the other rides, which is what alot of us actually love about Disney!
Worst of all, though, is that dead pavillion between UoE and Mission Space over in Epcot. Yes, the old Wonders of Life. It's the worst because it just sits there, offering us nothing anymore.