Slimes FTW!
Definitely, low-level slimes.
In 3e, you had Brown mold as a low level hazard.
One of the nicer enhancements 3e has made is their 'saving throw against continuous effects', which allows for low-level monsters to paralyze, etc, without taking a party member totally out of the fight. A slime which paralyzes is now more viable at low level, as you can get a save each round instead of being locked out for 3d4 or whatever.
Definitely, low-level slimes.
In 3e, you had Brown mold as a low level hazard.
One of the nicer enhancements 3e has made is their 'saving throw against continuous effects', which allows for low-level monsters to paralyze, etc, without taking a party member totally out of the fight. A slime which paralyzes is now more viable at low level, as you can get a save each round instead of being locked out for 3d4 or whatever.