What's the best flashcard app? - anki or quizlet? or maybe even tinycards?

Well I would recommend quizlet because of many reasons. Here are why:

Modern interface.

Apps for iOS and Android that can be used offline. There’s even an app for Windows 10 Mobile, but I have not tried it.

Allows you to add images to words, and helps you find the images in the first place.

Has optional test modes: multiple choice, or type the answers.

Can read the words out loud to you in a pleasing voice with good pronunciation and accent.

You can even type in Pinyin, and it will read the Mandarin words out loud perfectly!

You can “star” words that are most difficult to you, and review only those.

Has flashcard decks that other people have created, that you can practice.

Allows you to import and export data.

Resumes where you left off in flashcard sets when you go back to review them.

Here is why I would recommend Anki:

Has an app for iOS, Android, and Windows 10 (including Windows 10 Mobile), and a desktop application for any version of Windows. These can be used offline. There’s also a web app that can be used for practice on any platform with a web browser.

After each flashcard, you can file it into one of three review decks: Soon (<1 minute), Good (<10 minutes), or Easy (4 days). These decks allow you to practice the hardest words (Soon) most frequently.

Allows you to add images to words.

Allows you to add voice recording of words.

Has flashcard decks that other people have created, that you can practice.

Allows you to import and export data.

I would recommend Tinycards. I've been using them forever and it's just the best. Here is why:

The app enables you to 'favorite' good decks, to keep them visible while you try hard to excise rubbish ones from your memory. Of course, you can also make your own. The interface for doing so is straightforward, with you loading images from your device, and tapping in answers.