Ah, Oozes. Man, did I hate oozes, jellies, all that crap when I was a new D&D player.
I mean really, who wants to strap on a sword or a spellbook and head off valiantly in search of adventure to get kakked by a friggin blob of pudding? Not entirely heroic.
Ya can't hack them to death, and torches might cause damage, but you'd better have a few extra lit and ready if you don't want to risk putting your only light source out trying to kill the ooze. Just the thought of these creatures makes me shudder.
I do have a favorite, though. The Gelatinous Cube. I love these things as a DM. I love how nervous PCs get when the dungeon floors and walls are just a little too clean. When there's a lack of organic debris or rat droppings. Characters start checking behind them, and down every corridor looking for any sign that a nearly transparent wall of digestive Jell-O. Sometimes, as in this illustration, the cube can be given away by the spooky suspension of the metal equipment of some poor adventurer in the middle of the monster. I totally dig this monster, and I use it to mess with new players who aren't so steeped in D&D lore that they expect it.