I use Kanjidojo to learn the writings of kanji, you can build your own lists or select jlpt, grades etc. I still use Renshuu to learn the readings and meanings of kanji. With kanjidojo you can write the kanji on your phone and decide if you remembered perfectly/good/meh/not at all so you will re-write the kanji sooner. My advice is also to understand the meanings of radicals and imagine stories (or read the mnemotechnics of other renshuu users) and it will become harder to forget kanji.
Same as ポールおじちゃん for me. And if you can afford the pro version, it's even better. The kanji crosses exercise in particular is really one of the best exercise I've seen.
But even without the pro version, just the onyomi and kunyomi vector will take you very far. The writing exercise is also good at the beginning even if I don't like it for later because it kinda give you the solution by showing the correct too early. I prefer app that only show me the solution at the end (like JA sensei).
Wanikani is a website and I think it's decent. It's paid past the third level, and teaches radicals, kanji, and vocab with mnemonics. Thought, like the above people said, renshuu works just fine.
