Here are some of the apps I use for edits:

Write Behind:

-This basically lets you do what the app icon says. It's possible to do this in Superimpose and PicsArt as well, but with those two it's a tad bit more complicated if you don't have PNG's. /Not Necessary/

Polarr

-This is basically a filtering app, it really is helpful with brightening photos/changing tints.
/Probably Necessary/

Superimpose

-This app does a multitude of things. It's basically for layering and it's really great to use when you're making a cover that involves a PNG (a photo without a background) and it's also helpful for making them. HOWEVER, due to the fact it costs money (.99) I guess it'll have to be
//A Probable Necessity//

PicsArt

-It's a lot like Superimpose, but you can add text/filters/color changing. I wouldn't recommend it for text, but it's up to you.
//Either Or (Superimpose or This)//

Phonto

-This app... Is a godsend. It's amazing for fonts, downloading fonts, and you can even make templates and for those who write Warriors fanfictions, you can add a circle image onto a background. You can also box your text, put a diamond around/on it, whatever shape you want to chose.
/Necessity/

Real Colors

-This basically helps you generate a palette of colors from your picture; which is really helpful if you're trying to find the perfect color for your text. However, if you don't get this, it's always safe to use black/white/neutral shades.
/Not A Necessity/

We Heart It

-This is basically great for finding covers if you can't find any from Google. Tumblr is also a good alternative.
/Up To You/

Glitché

-Like the title suggests, this is basically for glitching out your photos. It also makes glitchy gifs if you want to add some to your story. Even if it didn't cost money, I'd still rate it
/Not A Necessity/

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These are just some of my favourites!!! Please, go ahead and comment some of your favourite apps!