I'm looking for a challenge, does anyone have any ridiculously hard Classical Pieces on bass.
are you willing to learn/tack multiple parts?
https://www.youtube.com/shorts/v4tLpj22vyc
Almost any really interesting piece that's physically possible will warrant multiple parts.
if you're willing to use a 6 string bass, your options grow quite a bit. i'd definitely say the fugue from BWV 1001 is one of the most difficult (but definitely playable) transcriptions for 6 string bass. i've played it in pieces but never recorded the whole thing.
i'd definitely advise against finding some lightning fast piece by paganini or whatever. obviously i love that music, but if i'm going to put in hours and study a piece i definitely would crave more substance. some good examples are any of the solo violin partitas by Bach. they are playable and many of the movements are actually quite simple. same goes for cello suites, even easier.
i'm a bit conflicted on what you mean by ridiculously hard - i have a feeling you mean speed and shredding (bach to paganini

) - but i would say "hard" comes in so many varieties, particularly when transcribing from one instrument to another. taking a chopin nocturne for example, and reducing it down to a bass note, perhaps some chordal tones, and the melody... would be amazingly difficult and very beautiful if you can execute it well.
anything that involves being true to an orchestration seems to be the most difficult. if you can take a piece of Berlioz, and do it justice while changing the orchestration... that's really difficult and amazing. same would go of course for anyone else known for their orchestration - Brahms, Stravinsky, etc.
you could always look up some insane etude or sonata by ysaye for violin. but i think so much more satisfaction can be had pursuing really exquisite music that also happens to be difficult (no shade to ysaye


)
remember that the majority of classical repertoire can be had for free in imslp.org. if i were you i'd scroll bach, beethoven, mozart, brahms and look for things that you think you can make work. definitely don't shy away from lesser known names or peices in multiple parts, but those names carry such authority for many valid reasons.