The obvious choice is iMovie. It comes with the iPad, and it's free.
But it's a huge step down from something like Adobe Premiere Pro and doesn't do some things that I want from an editor. The color correction tools are weak, and the app doesn't accept LUTs (which are used for color grading).
LumaFusion is not free. It's $30, but if you've spent hundreds of dollars on a drone dSLR, then this is a drop in the ocean, and it gets you a solid app that's capable of a lot.
First off, it can handle multitrack video editing, which for anything more than a simple video is a must. And it handles the demands easily, no matter whether I'm using HD video, 4K video, or even 5.4K video.
I can trim, edit, color correct, add audio tracks, add titles, and much more.
LumaFusion also comes with a built-in "Lock and Load Stabilizer" for correcting shaky footage. This feature was recently added to the app, and I'm very impressed by the results from it.
I like to slow down and speed up drone footage a lot, so the ability to work with frame rates from 18 to 240 frames-per-second, with fantastic results is important to me.
Another LumaFusion feature that I like a lot is the ability to edit video directly from external USB-C drives. This means that I'm not having to fill the iPad Pro's storage with videos.
There are also a bunch of other features in LumaFusion such as green screen and chroma keys, which might be of use for you in your workflow.
I'm impressed by what LumaFusion has to offer, and for $30 it has a lot to offer both the amateurs and professionals alike.