I think the MM1 hydra has to take the ultimate prize for worst monster. This is the ultimate boring monster. Every stereotype of solo monsters and 4e "grind" is traced back to this culprit. Fortunately there are a bunch of other hydras that are cooler.
Black dragons are pretty boring. Low damage, and an ability that gives them full concealment. Really the only bad monsters I'm thinking about are solos. I can't think of any non solos that are that bad. Boring sure, but not terrible.
I don't have a problem with most insubstantial creatures - sure you do half damage to them, but they also tend to have very low hitpoints to begin with, so it's mostly a wash. Really a reward for characters that invest in magic items, powers, feats, etc that affect insubstantial as they wind up being the star that fight. Granted some of the insubstantial examples people were posting previously were not lurkers, which is bad design, so ok I'll agree there with earlier examples.
As far as the low damage but going through walls and regenerating, usually I don't use that to be a grindfest... I use it to have the monsters run away, regen to full and then spring out at the party as they are in the middle of their next encounter. Making them even more hated and feared, and thus enhancing their value and coolness. Yes, I can be a bastard.
I don't really have much of a problem with most monsters... I see them as tools, some to be used more sparingly than others. I see more a problem with bad encounter design than bad monsters... low damage monsters that are really hard to kill should be combined with high damage monsters, not encountered by themselves for example. For example teamed with an elite controller that enhances their abilities.
A low damage causing, sack of hitpoints, that does no meaningful conditions is "boring" but it's also nice to have a few of those when the encounter also includes really complicated monsters so the net tracking I have to do as the DM is not overloaded. It also puts more shine on the "interesting" monster, which for dramatic reasons is sometimes nice.
Here's a challenge - are there any monsters that are so utterly bad that you would never use them EVER, under any circumstances? I can't think of the top of my head a single non solo monster in any of the monster manuals that I wouldn't EVER use, given free reign to design the terrain and additional monsters that form the overall encounter.
The real problem are encounters where the party focus fires all their encounter powers on all the interesting monsters in the first few rounds and leaves the boring monsters, often still with full hitpoints, for cleanup at the closing rounds... when they are left with at will powers. This is something that comes up a lot, and is a challenge I have to anticipate and deal with regularly as a DM.