Fallen a little behind on the challenge, but here’s the 23rd: my least favorite monster.
There are plenty of monsters I never use, and don’t care for; there some monsters I use and regret introducing.
Starting with the second type — monsters I regret using — the answer there is vampires.
You’d think they’d be great monsters — they are a classic, a staple of horror movies, Halloween, and folklore. There are lots of weird variants so if you don’t like the (now) vanilla Bram Stoker version you can do the Philippine flying head with a tail of guts or any of a number of different things.
My problem with vampires is that they take way more planning than most monsters, and their vulnerabilities and dependence on a coffin make them fairly easy pickings for a party of adventurers that is tough enough to face them in combat. So, due to either my incompetence as a DM or my player’s excellence (or both), the vampires that have appeared in my campaigns have always been duds, and never prove to be as challenging as they should be.
Regarding monsters I just don’t like and never use, I spend a lot of time thinking about them, but having just had a question about dragons I am reminded of the “good” metallic dragons which I kind of hate.
In principle, a good dragon could be a problem or foe for a party since they will have their own agendas and possibly stand in the way of the party’s goals. But in my experience, mainly as a reader of adventures more than as a player or DM, good dragons are usually handled as powerful NPCs that railroad a party or just show off how awesome they are. There is no reason to have a good dragon in an adventure unless it is there to be defeated. Yet when I see them in old magazine articles, published adventures, etc. they are usually just “Mary Sue” NPCs that are there to hog the limelight and waste my time. So screw good dragons.