I've tried a lot of the usual recommendations and none of them meet my needs. The best one for my purposes is Photo Editor by dev.macgyver.
If you are simply looking for an app that adds filters and effects to photos, this isn't it (though it has some of that) but is a powerful (mostly) full-featured photo editor. I'm accustomed to Adobe Photo Elements and many of the features can be found in Photo Editor. It had a slight learning curve but it's worth getting to know.
Full disclosure: I've been using this app for the past four years so it's possible there are new ones out there that are really good as well. But since I know how to use this one and it does everything I want, I've not gone looking at other apps.