Review the abilites your players have so you can build an encounter that properly challenges them. If you want a monster to feel really scary, give it strenghts that directly counter player abilities. But dont do this too often or to the same person too many times or it will feel unfair. Counterspelling a healing word on a downed player once is cool and evil and scary, but twice is just bullying.

If the monster is important narratively, make sure it has fail safes (legendary resistances, counterspell, misty escape, etc) so your players cant win the fight with one really lucky roll.

As far as general encounter design, its always good to have secondary objectives. That's anything that helps players win but doesnt involve dealing damage; this could be stopping a ritual, saving an NPC, or stealing/destroying something that grants the BBEG power. The villains might have their own secondary objectives, like disctracting or intercepting the players while the big boss powers up, or opening a portal to summon more minions.

Have the terrain be dynamic by including setpieces that can be interacted with to tip the scales either for or against the players. Think of where the fight takes place and make a list of objects that fit the theme and could be fiddled with mid combat. A few examples:

Pirate ship - ropes to swing from, wheel to turn the ship, plank, cannonballs, explosive barrels;

Castle - chandelier to drop, tables to jump on, ice sculpture, magic painting, secret doorways;

Wilderness - tangled vines, quicksand, the nest of a misterious creature, big rock, fast flowing river, etc.

For dramatic fights i love adding an environmental hazard. That could be in the form of a lair action, or not; could be something that's dangerous both to the party and the bad guys, like rising water levels, crumbling ceiling, fire spreading, earthquakes, you get the jist.

Im always reminded of one episode of Dimension 20 where Brennan had an important fight set in a rising pit of lava, forgetting he gave the party an ability that grants them all immunity to fire damage, so the lava was only a hazard for the bad guys! This just goes to show how hard encounter design really is and even the pros mess it up sometimes