Going with most disappointing as in a title I was actually really excited about and really disappointed by the execution, the ones that come to mind first(probably due to having done the Wildstorm thread on these boards recently) are Morrison's Wildcats and Authority.

Wildcats just came off a very strong, though very unpopular run (due to DC marketing having no clue how to market the title.)
To recap:
Wildcats volume 1: Image comics, Jim Lee and his pal Brandon Choi create a superhero team in the vein of X-Men with the twist that the heroes and villains are members of two alien races at war who crashed on Earth centuries ago and inspired myths and legends. Choi is not a great writer, so Lee hired James Robinson, Chris Claremont and later Alan Moore to improve that part of the title.
Wildcats volume 2: DC, started as a vehicle to showcase Travis Charest art, but quickly turned into one of many Joe Casey/Sean Philips coops. The alien war has ended, the heroes have doubts whether they were even on the right side of that war and now have to move on. The series deals with cleaning up the fallout of the war.
Wildcats volume 3: One of my favourites. The remaining Wildcats decide to use their alien technology to improve humanity starting with producing batteries that will last forever. A small invention that has large consequences.

WIldcats volume 4: Morrison writes a single issue that is incoherent, reverts a lot of things back to issue #1, has a couple of pages that have messed up colouring (which may have been intentional or not) and gets bad reviews. Morrison throws a hissy fit and drops the series after a single issue.

Authority: After Ellis left, this was another series that was looking for an identity. Millar was interesting if too invested in shock. Brubaker was interesting. Morrison had the amazing Gene Ha on art and spend the first issue with a guy making breakfast with his estranged wife. Reception and response were pretty much the same as for Wildcats (though I think he actually did write a second issue here). This at least was more coherent than the first, though the main premise "What if Superheroes were in the real world?" seems to as uninspired as you can get.

Fast forward 10-15 years (which includes Wildstorm editorial that has no clue what's happening in their own titles leading to massive continuity errors between various titles at the same time, followed by rebooting the whole series as a post-apocalyptic setting, which has a Wildcats by Gage which was ... fine, moving the Authority to the DC new 52 universe as Stormwatch, an actually great Midnighter (and Apollo) series by Orlando) and we get Ellis getting things back on track (let's see if they can keep things there. The first spin-off was not very promising.