Don't Take Away The Player's Turn
If you want the game to be fun, and you want the players to pay attention, don’t take away their turns with spells and effects. To a player, getting stunned for one turn is like being told they can’t play the game for 2-20 minutes, depending on the system and encounter. They’ll try to stay engaged, but eventually their mind will wander, and you’ll have an uphill battle to regain their attention even once they’re back in action.
5E has gotten pretty good about this. Effects that petrify you do so slowly, so that combat (which rarely lasts more than 3 rounds) is usually over before you’re fully incapacitated and your friends have to decide what to do with their new statue. Most of the Save-Or-Lose spells have vanished or become very easy to resist. Many effects which previously stole your turn (like being grappled) now only create an interesting handicap to deal with.
But there are still a handful of effects like these: Sleep takes a player out of the fight at low levels, and though they can get woken up, they’ll still likely miss a turn. Tasha’s Hideous Laughter and Hypnotic Pattern outright incapacitate characters and override their turns with boring saving throws until they succeed. Fear moves the player’s character around, but at the cost of at least one of their turns. These spells are in the game for the players to use, because they’re almost always using them against you, and you have multiple characters to play.
Instead of taking away the players’ turns, you want to CHANGE what they can do. Spells like LEVITATE and SLOWNESS limit players’ options, and force them to make interesting tactical choices (the cornerstone of a satisfying strategy game). That levitating half-orc needs to throw javelins, or find a way to push off of other objects and turn into an orcpedo, instead of just charging in with his axe. Slowed players need to coordinate their efforts to get to the spellcaster at the back while his buddy tries to block their slowed-down movement by placing obstacles in their way. These fights are much more memorable than that one fight where Jerry was knocked out, got bored, and distracted everyone with cat videos.
Good spells to throw at the players break up their routines and prevent them from boring themselves by doing the same thing in every situation. Other times, they’ll add an interesting twist to the encounter. Here’s a handful for consideration:
COMPELLED DUEL can create a tactical situation where the party’s usual meat shield/assassin is encouraged to fill a different role, forcing other players to deal with the general fight without them.
PHANTASMAL FORCE is always fun-- it can make an extra blockade that a character can choose to ignore (at penalty). Rather than taking options away, it makes one less appealing (but more badass). Same story for effects like Spike Growth that create barriers you can pass at a penalty.
BANE is a little lackluster-- subtracting from rolls is not a very strong or interesting effect-- but the players will hate that enemy with a fiery passion, and deeply prioritize them as a target, because players hate feeling handicapped.
On the other hand, there are some corner cases that skirt the line:
DOMINATION is tricky and requires a judgment call. At the very least, don’t take direct control of the player’s character away from them-- ignore that optional half of the spell entirely. Ask the player to switch objectives and come sit on your side of the table until the spell ends. The right player will have fun playing turncoat; but if you think the player will have trouble separating themselves from the stakes their character is facing, don’t take away their agency.
ENTANGLE/BLACK TENTACLES are tricky. They can force a brute melee character to rely on their underused ranged attacks, but if they don’t have any, failing saves against these turn into lost turns, and if their ranged attacks really aren’t any good, they can be just as unsatisfying as a lost turn.
When to break this rule:
You can absolutely break the rule and throw these effects at the players, but when you do, it shouldn’t be to make the game harder. A game is difficult when the choices it presents require strong logic and accurate guesswork to navigate. A game that prevents the players from making choices at all isn’t difficult, it’s boring. Rolling against Hold Person is the world’s slowest game of craps.
If you’re a hardcore simulationist, and you can’t imagine a world in which the world-ending cultists are tossing around less-than-totally-debilitating effects, stick to damage spells. They’re more effective than save-or-lose-your-turn spells anyway.
Personally, I try to only throw save-or-suffer effects at players as a parting gift. If combat is going to be over before the player misses their turn, either because the enemies are at low HP or are about to flee or reinforcements are going to show up, then it's a fine time to throw out a disabling effect. The reason for this is that-- as long as it’s not a concentration spell-- the disabling effect becomes the next scene in the game. Tension de-escalates from combat to solving the problem the dying/fleeing villains created.
Even when a player is knocked to 0, or dying, or dead outright, I try to give them a turn-- whether that's interacting with near-death hallucinations or picking one ally to give the help action to as their spirit drifts away towards the afterlife. Though sometimes, in these cases, a player might be too shell-shocked to want to finish the fight-- or too busy rolling up their new character so they can jump back in ASAP when it's done.
DON’T use spells, monsters or effects that stop your players from playing the game.
DO use spells, monsters and effects that make them play the game differently.
Knock them down, take their swords for a while, concuss them, transport them to the pickle dimension, but don't ever tell them they don't get a turn.