Well of course I need to tell you that PicMonkey is a great photo editor! Otherwise, it depends on what you want to do. For high-end photography that needs large print resolution and complex features like luminosity masks, Photoshop and Lightroom are great and take you a long way. If you're mobile-only, a similar tool like Snapseed is a good one-stop shop for professionals.
But in general, you want to look for an app that does really well with these features:
Exposure. Exposure is the result of three different variables that determine if your photo is underexposed (too dark), overexposed (too bright), or evenly exposed. These three variables are sensitivity (ISO), shutter speed, and aperture. If you photo is too dark or a bit blown out, you’ll want a tool in your app that lets you adjust its exposure without losing detail or making it look washed out. If you want to get advanced, look for a tool that provides you with a histogram and then bonus points if you can edit the histogram graphically.
Highlights and shadows. These tools let you adjust the bright tones (highlights) and dark tones (shadows) in your image separately. You can pull out some detail in almost black rocks by lightening the shadows a bit, and reduce the glare in a sky or face by darkening the highlights a bit. But make sure the tool goes both ways (brighten and darken) for these two. Brightening the highlights and darkening the shadows extends the dynamic range of the photo and is as critical a tool as is vice versa.
Vignettes and blur. These tools are useful for putting the focus on the subject of your photo and removing distractions, either by adding darker edges with a vignette (like Urbane in the PicMonkey mobile app) or blurring the area around your subject. Many photographers look for camera lenses that provide a good bokeh, which is the term used for the quality of the background blur in a photo. A good blur tool will mimic this, and make your subject stand out a bit. Vignettes are important for ensuring your viewers eye doesn’t wander out of the picture. Bright edges will draw a viewer to the corner of a picture and then wander out of frame. A subtle use of a vignette, therefore, can contain the viewer completely in the scene and on the subject.
Sharpening. Sharpening makes the details in your photos look crisp and clear. It’s a good go-to edit for most photos, and especially useful on images that look slightly soft or blurry. Clarity may get you a similar result but can lead to more contrast than is desired. Look for both of these tools.
Black and white effects. Black and white gives photos a certain feel. It’s classic, sophisticated, artistic. Using a good black and white effect is an easy way to transform your photos, but it’s also a great way to save a mediocre picture. Black and white removes the distractions that color creates and can make a slightly confused picture suddenly have a clear subject. It doesn’t always work, but with a good B&W or monochrome effect like Mercury in PicMonkey mobile, you can experiment with a picture that is not quite working as a color version.
Once you’re comfortable editing and ready to grow your skills, I recommend using curves, spot healing with tools like clone, and experimenting with textures. I wouldn’t brand these features as “basic,” but they’re incredibly useful.
Also, opt for apps that allow you to apply edits, effects, and textures to specific areas of your image, instead of having to apply them to the entire thing. Small, painted-on effects can bring out local colors or details that give the overall photo much more depth and character.