As others have said, the edition's the thing. For "old skewl" I'm prone to agree with the owlbear or ogre for any group.
The troll, as Chris_Nightwing points out, is brilliant for the new players [and/or 1st level characters] for that precious "OH $#!T!" moment when they just can't get it to stop attacking. But I personally wouldn't use a troll is you don't have, at least, a mage [or any other PC] with a fire [or acid] spell/equipment they can use or some environmental resource (a nearby campfire, a lit torch in a sconce, a pool of oil that the PCs can dip stuff in and then frantically try to get their tinderbox out to light it! ...
<waxing nostalgic> Aaaahhh. I love the smell of panicking party members in the morning...</waxing>
ANYwho...some resource that a smart/crafty/ingenious player can use/grab to do the oh-so-necessary fire damage to stop regen.
Other suggestions, a "Bargle": Low-middling level MU or Necromancer or someone that has spells/magics the party doesn't or hasn't seen before. A fireball or lightning bolt against 1st level PCs also makes for a wonderful "Oh S--t!" moment. You don't have to KILL them with it. Say, the evil wizard uses it on the cavern/chamber ceiling or doorway to bring it down. The party knows this guy's got "big guns" and [should] run like hell.
Low-mid-level (like 3rd to 5th, if your party is 1st) Illusionists are GREAT against low-level parties if used creatively...and the PCs always seem to get so ticked when they turn invisible and flee to fight another day. Insta-recurring Villain/Budding Party Nemesis.
I know it's become commonplace with certain editions, but I am just
so anti-"slaying baby/mini-dragons" at 1st level. Hate it. Hate everything about it. Leave them out or make them something so impressively huge and powerful the party has no option but to flee in unabashed panic under the overwhelming pressure of the creature's fear aura...which by certain editions' books, I believe a 1st level PC wouldn't have any choice but to do. Fight and BEAT one?! Forget it.
A medusa is tough! Could be a great fight, though. Especially if you have some sneaky/smart players that will come up with some ambush, tactics or traps. Though I'm kinda a softy with low level characters (and new players!) when it comes to those kinda "permanent badness/insta-death" creature. So I'd prolly make sure there was some story-based way to turn any "statued" PC back if the party was successful. Easiest thing to do, a.k.a. "the Saturday morning/after school cartoon solution" (which not everyone enjoys) is: If you kill the medusa then for anyone she turned to stone, the effect just wears off and everyone's fine....A mystic fountain of stone to flesh potion in the vicinity also works well.
That's mostly what I have. I'd stay away from aerial creatures with a 1st level party...and probably nothing (in old school terms) above 5 HD.
Good luck and let us know what you go with/how it works out.
--SD
