Aaaargh! is the #1 homebrew spell on DnD Beyond. It has a whopping 213 points (+243, -30) and 487 adds. It is available to bard, ranger, sorcerer, warlock, and wizard.

Basically, it commands the target to scream for 1 round, preventing V spellcasting and deafening creatures within 10 feet.

I'm left scratching my head. This is basically a limited version of the command spell (where the only command you can give is "scream!").

What makes it so great and worthwhile in your opinion?

It creates a local Silence zone by deafening creatures - the target plus those within 10 feet. It can therefore assist your rogues by creating a deafening distraction so not only does it cause a stir making guards etc look away, it allows the sneaky rogue to come up and stab from behind or sneak passed them safely. It can instantly stop a mage from being able to cast spells for a round and doesn't require a saving throw (it lists Wis Save but it a spell's ruling for use is in the descripton text not the tags used - the spell description details that you target a creature and it just takes effect - it cannot be resisted).

Unlike command, which is limited to clerics and paladins this spell is open to pretty much any magic user.

It's better to describe it as a one-round, no save, lesser version of Silence but with the added use of making a useful distraction.

It's also very gimmicky, what others think is "funny" to use and sadly people tend to upvote "silly" and "quirky" over efficient or balanced spells. If this was a videogame this would be the kind of spell some people would just go round a town and spam-cast this on everyone for the "lulz". Regrettably these will often get more upvotes regardless. This spell doesn't actually deserve the votes it has: the maker messed up (see above about the "save"), it's silly, it's usefulness is highly situational, there are better spells as part of basic rules, it's incredibly weak in all aspects and sacrificial. I say sacrificial because even if you use Subtle Spell Magic the spell description (which is not covered by Subtle Spell Metamagic as was confirmed in Sage Advice) requires you to shout at the target which will reveal you as the cause. If you made a distraction, t's great for others but you'd not benefit yourself because by shouting immediately beforehand it will be known you are the cause. so attention is drawn to you. You can use it in battle to shut down a mage for a round but frankly there are better spells you could use for that.

It's actually not a very good spell, mechanically, but it's absurd and gimmicky and "upvotes" is a very gut-reaction thing: it made them smile at the thought, it got added/upvoted. And this is why I ignore the voting system completely.

If there is no save, it is OP. 

I'm conflicted on the no save thing, because it does feel OP to shut down casters for one round with a single level 1 spell. But Silence doesn't have a save either and there is no way to be subtle about casting Aaaargh?! while Silence still has plenty of uses when sneaking around.

I think it's meant to have a Wisdom save, as the spell tag indicates, but it's true the descrition doesn't mention it. If there's no save, it's even more unbalanced. 

I have my own homebrew version of a "silence" spell that targets a single individual. I hadn't published it before, but I just did. It doesn't deafen creatures, and for balance's sake, isn't very good against spellcasters. Here it is for comparison's sake:

https://www.dndbeyond.com/spells/23585-hush