I like to throw a few cockatrice in with some carrion crawlers as a staple low level encounter (usually when they have something to do later in the day, making the 24 hrs paralysis a major problem.
Haven't seen anyone mention zombies.
I know people burnt out a bit on zombies as they saturated popular culture years ago, but they're ability to roll saves to stay on 1 hp really threw my players the first time they faced some in 5E, at first they fought there must be a head zombie that they had to kill first.
(Helped by the fact that for a second a forgot about the save, so just after I said the zombie fell, I remembered, did a quick roll and then described the zombie getting back up. It was one of those times that a stuff up added a little bit of detail that just really enhanced the moment.)
For me, the Vargouille was a monster I always loved, going back to Planescape but I don't remember ever using them, but recently I got a chance in 5E with a new campaign kicking off.
The opening session was at a small carnival with each PC competing in various events, the fighter had two rounds of 'first to 2 hits' type combat challenge, the first was a noble (who had already been 'kissed' by a vargouille The previous night and was looking pale and sickly) The player won the fight but lost against his next opponent, a thug. Later that night at the tavern the noble was there looking worse, he starting abusing the player calling him a cheat, I was hoping to have a fight start to maximise the shock of having the noble's head tearing off and sprouting wings mid battle. Thing was, with all the set up of describing the guy as Ill the player felt sympathy and had no interest in the attempts to provoke, in the end I had the noble storm off up a flight of stairs which still made for a dramatic moment when it happened.
No one in my group had encountered them before, so I was happy I took the time to set up the scene and then have a pay off, rather than just have the characters attacked by a bunch of flying heads in the woods or whatever.