I use Procreate, Sketchbook, and Clip Studio Paint. They all have their strengths and weaknesses.
Procreate is perhaps the best interface for people who just want to get lost in drawing/painting. It was designed from the ground up as a mobile app so it has a very intuitive UI. It also has some great brushes which are very easy to customize. Most importantly, Procreate has some excellent "professional" tools yet the app is designed in such a way that they never clutter up the experience or get in the way. If you want something that is powerful, elegant, and streamlined; Procreate is your best option. It's also amazingly well optimized with very snappy performance (though, to keep things fast, there are some layer limitations depending on your RAM and the resolution of your project).
Clip Studio Paint is essentially a desktop app that has been converted to the iPad without a whole lot of tweaks. It is incredibly power, giving Photoshop a run for its money in the breath of features and options for customization. There's simply so much you can do and was you can customize the experience. However, CPS can feel a bit cluttered even on a Desktop PC/Mac so it feels downright exhausting on an ipad. Simple things like adjusting the size/opacity of a brush simply take more time than they need to in CPS and you're constantly swimming in menus. The program also doesn't feel nearly as well optimized for the iPad. Drawing is snappy but I find the zoom/rotation to be a lot more sluggish than Procreate and I also frequently end up making accidental strokes when I'm just trying to navigate. That said, CPS has some AMAZING features that can save you a ton of time in the long run. I really can't overstate how useful things like vector brushes can be for some kinds of work.
Sketchbook Pro is an interesting app on the iPad. Back in the day, it did for desktop tablet art what Procreate did for iPad art. It was a streamlined program cleverly designed to work with a drawing tablet and stylus. The UI was very un-cluttered and some of the tools are still quiet amazing (you can zoom, scroll, and rotate all by holding down a single button then go back to drawing by releasing that same button). However, some of the things that made it so useful on a PC/Mac are less valuable on an iPad (for instance, using a touch screen you can already easily rotate, scroll, and zoom using your fingers). Perhaps more problematic is the fact that Sketchbook has started to fall behind when it comes to features, largely because it has since been converted to a free app and, as a result, seems to be getting less and less improvements. Over the years Procreate has included the ability to include clipping masks or use a liquefy tool while Sketchbook still lacks these very helpful features.
All in all, the app I use depends on what I'm doing. If I want to just get lost drawing or painting then I'll use ProCreate on my iPad or Sketchbook Pro on my Desktop. If I need to make something very complex (design a poster or comic), then I'll use CPS (or Photoshop).
