Best" is kind of tricky here because it depends on what matters most to you. Anki's algorithm is legitimately solid and you're right that it's hard to match, but there are trade-offs you might want to consider. If the dated interface is actually slowing you down or making it frustrating to add cards, that's a real problem - no algorithm compensates for an app you dread opening.
What's your main pain point right now? Are you struggling with adding new vocab efficiently, or is it more that the UI just feels clunky compared to other tools you use? That makes a difference. Some people swear by newer apps like Quizlet or smaller alternatives because they're faster to use, even if the spacing algorithm isn't as mathematized. Others stick with Anki specifically because you can customize the algorithm itself if you get into it. There's also AnkiDroid for mobile if that's part of your workflow - some folks find that changes the whole experience.
For German vocab specifically, are you doing anything beyond just vocab cards, or are you mixing in reading comprehension, grammar drills, anything like that? Some apps handle that stuff more elegantly than Anki does, which might be worth it even if pure spaced repetition is less optimized.