Following the lead of Monsters and Manuals: Top 10 Monsters, here are my Top Ten D&D Monsters, in no particular order:
- Gelatinous Cube: I love these guys. They’re creepy as all get-out, particularly when they’ve got a partially digested skeleton or something suspended in them, they’re not so dangerous as to be unfair and they’re the perfect accoutrement for that oubliette….
- Purple Worm: It’s a worm big enough to swallow you whole. It can come at you through the dungeon wall. And it’s purple. What’s not to love?
- Umber Hulk: I just like the look of them, back in AD&D 1e. Mandibles are scary. The 3rd edition version just looks like a bug missing some legs. I can take or leave the Confusing gaze.
- Cockatrice: Stoning is an awesome ability, but I’m not a huge fan of gaze weapons, so I like this guy better than the basilisk. Did I ever tell you about the time I used Telekinesis to hurl a black pudding at a cockatrice?
- Troll: One troll on the wall, on the wall, one troll on the wall,
if one of those trolls should happen to fall, Two trolls on the wall on the wall…. - Green Slime: it’s a horrible way to go, and a really useful weapon against other monsters.
- Golem: they come in a wide variety, and they can stand there century after century waiting to bash in the head of the next adventurer to come through the door.
- Liche: I never actually used these that often, but the fear of them was so strong that I once had an orc with a couple of faintly glowing gems held in front of its eyes bluff a party into retreating by advancing on them from the down the dark corridor. For the rest of the campaign, players would tease each other by making a holding gems in front of their eyes gesture and saying “Run away! Run away! I’m a liche!”
- Balrog: for some reason Balrogs, and not dragons, were the ultimate bad-ass monster in D&D to me.
- Dinosaurs: Breathes there the man with soul so dead
Who never to himself hath said,
“I’m fighting a dinosaur! With a Sword! Coooooool!”
Tales of the Rambling Bumblers 
Smaug was killed by a single arrow, whereas Gandalf struggled with the Balrog for a week (and sort of died.) Which ass is badder?
On the other hand, Gandalf was able to defeat a Balrog solo, but recruited a team of 13 dwarves and a hobbit to face Smaug. If he had it within his capacity to defeat Smaug, it was criminally irresponsible of him to inveigle the others into his scheme, and even worse to leave the dragon to laying waste to Laketown.
Gandalf’s morality and motivation are beyond mortal ken. In any case, I think it was more that a team of dwarves was set on facing down Smaug, and Gandalf recruited one hobbit to help them, while he went off on a crusade against the Necromancer. Smaug only actively threatened Laketown once those pesky dwarves disturbed his sleep. In any case, if you want to sue Gandalf for criminal negligence, be my guest.
He had a whole team with him against the Balrog. They just weren’t any help, so he told them to go on.
(Again