Highly recommended.
Anki is my favorate SRS for two reasons. It is probably the most customizable SRS app out there, and it's 100% free (on Android and Desktop). If you know a little html, there is no end to the ways you can customize your flashcards, and even if you're technology-challenged, there are groups on Reddit that can help you figure out the basics of how to personalize your Anki decks.

https://apps.ankiweb.net/

Recommended.
I like Memrise because it is user friendly, and there are many high quality user created courses available for free. I don't particularly like the spelling quizzes, but Memrise is a good option because of it's ease of use.

Recommended.
I only used this app for about two months. I generally liked it, although I did get bored of it after a while. The really interesting part about this app is that they don't base their flashcards around translations, but around fitting words into a full phrase, with is a much better method. This is a very simple way to get exposure to how words are used in the context of a sentence.
If you do decide to sign up for Clozemaster, I'd appreciate it if you use my affiliate link so that a small commission comes back to support my work. You can sign up here: https://www.clozemaster.com/?tap_a=19009-244996&tap_s=1671414-e9fac7

Not recommended.

I really wanted to like this app. The cool part about Drops is that they link each vocabulary word with an image rather than a translation. This is a great concept because it's never a good idea to filter your target language through your native language. This strategy has severe limitations, however, because many words cannot be depicted in pictorial form (how would you draw an image of the words "although", "if", or "probably"?)

I also find that they treat languages like cookie cutters, which does not work at all. For example they use one image for the word "grandmother" in every language, but in Thai, there are two distinct words for grandmother that are not interchangeable (one word for paternal grandmother, and another for maternal grandmother). Drops just teaches one of these words and ignores the other.

Tentatively recommended.

As I've mentioned before, you should absolutely be using lingQ for Comprehensible Input.

Now, as far as their built-in SRS system goes, it is fine but it's nothing special. The one really nice feature LingQ's SRS has is that they automatically create flashcard entries for every word you are learning in your other readings on LingQ. If that sounds like something you're interested in, then the LingQ SRS system can be of great value. I personally prefer to simply do my reading on LingQ, and get SRS reps on Anki.

If you do want to purchase a premium membership to LingQ, be sure to sign up through this link: https://www.lingq.com/en/fingtamlanguages/ (That link will give you a 35% discount, and I'll also receive a small commission.)

Follow your heart.

Some people just don't like technology, and that's fine. The downside to physical flashcards is that it takes longer, and you cannot attach audio files to them, but I've successfully used physical flashcards to learn many words.
When I was learning spanish, I carried a small pocket notebook everywhere I went, and I would

Pimsleur is not a flashcard app, but rather a collection of audio lessons based on listening and repeating. Hoever, they do incorporate SRS methodology in their system. I find Pimsleur to be a very good way to learn new vocabulary as a beginner. It is the perfect way to make use of down time while driving or exercising.

If you sign up for Pimsleur using this link, I'll receive a small commission on your subscription. Thanks! 😃
https://imp.i271380.net/c/2972185/1007036/11472