A long standing favorite of mine has already been kind of mentioned.
Goblins (or any other creature with hide as a bonus action) using ranged attacks while in a complex environment that provides lots of hidey-holes and ways to break line of sight. The constant game of hide and seek generally wears down the party pretty fast since most of my players don't think to ready attacks or hide themselves.
Another is the use of poison clouds or other such effects by creatures immune to the effect. I had a Vampire Lady whose throne room had "incense" burning, that was actually a slow acting poison. Or, another time, a cowardly wizard tried to lock the party in a room with a brazier, a flying mephit, and a bag of poisonous stuff the mephit dropped into the brazier.
There is of course a difference between defensive plans and offensive. The party is often the one "breaking in" to a monsters lair, and I've already got plans for an Mindflayer Colony where the Elder Brain uses their At-Will Wall of Force to separate the party and force them into 1 VS many fights against Mindflayers, pretty sure that one would be a TPK though.
Most of the ways you can split the party are incredibly effective, wall spells, teleportation runes, anything that breaks up their normal attack patterns and lets the monsters set the terms of engagement.
I had one creature which was a mix of an Aboleth and a Beholder that I used for an incredibly powerful foe. I switched their anti-magic eye for Mirage Arcane, then, I had the entire room the party fought in be an illusion create inside of the Boss' highly acidic tank room. As a legendary action the Boss could alter the room at will, opening windows that he could fire out of or turning the floor into spikes to damage them if they moved. Having 100% control of the environment and using it to make every choice painful and awful.
Having an enemy healer/buffer hiding behind a Sanctuary spell is pretty good. Hard to attack the guy whose healing everyone for tones of health if you have to make a high save every time.
Ooh, just had a thought about a group of high level casters (probably cultists) who cast banishment on a bunch of their allies. Then, part way through the fight, or when they drop, the get reinforcements in pre-determined locations. Or, they lure the party further into a room then drop concentration to pincer the party between two forces.
Flame Skulls or something similiar are pretty dang terrifying. Put them with a bunch of helmed horrors. The Horrors charge the party, lock them into melee, the Flame Skulls fly above and just lob a bunch of fireballs down. 4 skulls with 3 castings each is 96d6 damage that the players are probably going to take at least half of while the horrors are immune. Then, even if you defeat the skulls, if you don't remember the trick with the holy water they are back up in an hour, with full health. So when you go to leave, or when the skulls catch back up with the party, then they just make everything worse again, even if they have no fireballs left.
Of course, something I really want to do at least once, not really a strategy but a scenario, is get a player to agree to a "we both get a single strike" contest with an Ancient Spellcasting Dragon at the top of a mountain. You must accept the strike with no resistance. Dragon touches warrior (who else is going to try and kill or impress a dragon with a single blow or try and survive a single blow) and casts Dimension Door, taking both Dragon who can fly and warrior who can't an additional 500 ft out and away from the mountain. Dragon flies back while the warrior learns that I don't set a limit on fall damage and surviving 200d6 damage is very very unlikely.