I like certain D&D monsters a lot, and use them often when DMing. I use NPCs as most of the bad guys, but the occasional monster is pretty necessary, or you're not really playing D&D.
You will probably also notice that most of the monsters I like are pretty mundane. Beholders are not interesting to me. They're just a little too gonzo. I love using undead, so I hate clerics that are good at Turn Undead. That one ability pretty much makes an entire category of monsters kinda lame.
Goblins: Excellent bad guys for low levels. Adding in Worgs makes them even more fun.
Skeletons: I like having them as part of traps. Maybe they are sitting underwater in a partially flooded corridor and will jump up as the players walk through.
Werewolves. It's hard to use these monsters well in D&D since most people figure out what the bad guy is and know how to defeat it, but I still like them.
Wraiths. I like having them come through walls to attack. I give them a huge bonus to surprise the first round if they do this.
Displacer Beasts. Use fireballs to saturate the area. It's the only way to be sure.
Giant Spiders. Lots of webs to trap players and poisons to ruin their day.
Vampires. Also usually recognized ahead of time, but they are classic villains for a reason.
Wererats. My favorite were-creature. I tend to have one or two tougher wererats leading all of the normal ones, as otherwise, they are kinda wimpy.
Various animals, such as bears and wolves. I keep this pretty rare, and often have them show up controlled by another monster.