I feel like a lot of the martial classes have a lot of room with this question. They can describe cool fighting moves and how their weapon of choice does so. 

What do y'all say/how do you describe a final blow when it comes to spells? I know it can be really interesting but I got nothing!

I usually prefer martial classes myself but I really enjoy hearing Liam O'Brien describing his spells on Critical Role.  When casting he uses the spell components (V/S/M) to describe what his character does (muttering a few arcane words / slapping his hands together and then drawing them apart / drawing a ball of sulfur and bat guano out of his pouch...), it's just as evocative as any fighter talking about sword techniques.  You can also incorporate the scenery and other players into your action; maybe you're leaning around a pillar or cowering behind a desk, or you have to hurl the spell over your friend's shoulder to reach the enemy.

As for the effect on your enemies, consider the type of attack (AOE, direct...), where you hit, and the damage type. A fireball might detonate at the enemy's feet, hurling them across the room and leaving a still smouldering corpse slumped against the wall.  Or hit them with cone of cold, leaving them frozen solid with long shards of ice trailing away behind them, their arms lifted up in a futile effort to fend off the blast (for extra flavour you can walk over and shatter them afterward). For cantrips you might engulf their head with a fire bolt, or freeze their chest/heart with a ray of frost, or leave them twitching sporadically on the ground with a shocking grasp as steam rises slowly from their clothes...

For Warlock specific spells: Eldritch blast does force damage, so blast chunks of flesh off them or punch a hole in their skull. Arms of Hadar might wraps tendrils around an enemy and squeeze it to death, or just whip about in a frenzy and batter them to a pulp.  If you use Blight you might watch as their body withers and their skin bleaches, leaving a twisted corpse consisting of just skin and bone.

Really, just have fun with it.  :-)

Watch more anime for inspiration, cause it's filled to the brim with cool ways magic happens.

Armor of Agathys: "Just as the goblin rushes in to make his attack and swings his weapon, the warlock stands there unflinching with a composure as cool as the ice and frost that surrounds his being. With a quick lunge, the goblin connects their hit, but finds their blade stuck in the ice. They try desperately to pry it from the frost as a chill runs up their spine. It is soon after that the warlock shoots a cold, piercing gaze at the goblin, and an icicle shoots forth from their being and impales the goblin where they stand as it tears through their tiny green abdomen." 

For a neutral evil character I've melted faces off Ark of the Covenant style using acid, exploded heads with lightning, reduced a target to a withered husk with necrotic damage, burned someone alive for insulting me before the fight…

The general rule is forget about whatever first springs to mind, there's always something nastier 😈

Of course if you're playing a good character you might want to make it more merciful with clean kills and such; you can still describe how an eldritch blast bolt hits the target's heart, or the magic animating a construct or undead dissipates with a crack of opposing magics and so-on?

My DM asks that very question on any critical hit that drops an enemy.  Plenty of times my warlock has done some cool stuff with an EB coup de grace.  Crushing the chest cavity of a bugbear as its knocked back to its death, ripping open the scaly hide of a giant snake, the smell of ozone as the energy blasts a wight.

For spell damage without a to hit, if its impressive damage or a critical fail on a saving throw leading to death, the DM will ask for some description from the player.  We find it rewards a great roll, and is uncommon enough that when it happens it warrants the added commentary. 

Good idea in theory, but anime isn't really my thing :/

Any other types of media that you think might work? And I love the description you gave for armor of agathys!

Thanks! So, the only other thing I might recommend is to watch anything that's high budget animation, in general. Anime, cartoons, whatever just make sure it's high budget. Why not live action? Because animation captures subtle things and movements, and exaggerates them to make them a focal point without making it look off, and when a lot of money is put into it it usually means things are animated very well. Animation in general also does magic super well, and a great example of this is the netflix witcher series and the animated witcher movie that came out. Both of those shows demonstrate the same powers one live action and one animated, but one does it sooo much more detailed and cooler. 

But all of that comes down to your ability to be observant of these things. Your mind's eye has to take note of these things before you can envision your characters doing the same. It also helps to learn different ways to describe something, so reading more books and absorbing more literature in general helps with that. Then you can better communicate it! You can be flowery or to the point depending on the tone you wanna go for, but it's only something you become aware of after absorbing enough of that junk.