I love giants. I love dragons. I love to make up all kinds of giants and dragons. I even love to give growth potions to dragons and hurl them against the tarrasque (the party that did this was genius until they healed the tarrasque to get rid of the dragons).
I love using evil minions like kobolds, orcs, gnolls, trolls, ogres, etc.
I really really like ghosts and shadows and redesigned the psionicist to be a class developed by humans specifically to combat ghosts and shadows.
I enjoy mimics and lurkers above as well as rust monsters.
Oh yeah, rust monsters make the magic-user in the party feel manly and brutish:
I killed it with my dagger all by myself without magic!
Spelljammer let me throw in an ancient mecha as well as Barsoomian Martians. I translated the races from Star Frontiers into the game to appear as NPC races to make things interesting. I enjoyed using the draconians from Dragonlance in other, non-Dragonlance locations.
I created creatures that were designed specifically to use a different magic system, just to throw off the players.
When I got my copy of Fiend Folio, I used an Iron Cobra almost immediately. Besides that, I love golems. I think I have a golem for every conceivable material, including rust, darkness and mold.
I had a party once show up in a battle between blink dogs and displacer beasts. This give new meaning to the term crossfire.
In all of that, how could I choose a favorite?
I couldn't. I don't mean that I love every monster - that's not true. The rot grub and jermalaine were not my favorite. I also didn't enjoy the Lava Children from FF. Many of the various demons and devils created over the years have little appeal to me.
So what is my favorite monster? My favorite kind of monster is the next one.
I know. You see what I did there. Really, I'm not trying to be cute or clever.
It's just that I never get tired of acquiring monster books. It doesn't have to be D&D. I've learned entire RPG systems in an attempt to use some of the monsters in the bestiary. Just give me more interesting monsters.
Interesting varies from person to person, but you know that many of you buy monster books for the same reason. The players can recognize anything after seeing it once. Sometimes, they recognize it faster than you can say Roll for Initiative. A new monster book has the chance to surprise the players and be really cool all at the same time. More than that, it has to be tweakable. Otherwise, your clever players will read about it in their personal copies of the same book you are using.
That's the reason I'll get Tome of Horrors 4 and Synnibar someday. It's why I have worn out every copy of the Random Esoteric Creature Generator. Everything may not be useful, but something will be beneficial. When you have to generate new threats, you can never have too many creatures to inspire you...
Again, not trying to be philosophical, its just that there always a good monster out there waiting for me to find/create it. Because I'll never discover/generate all the monsters, I really can't say that I have a favorite monster, except for the next cool one I find. Is that gamer ADD? Probably. I wouldn't have it any other way.