The thread's kinda dumb because everybody comes here with their own limitations, expectations and tastes.
Frankly, the most difficult bass song ever SHOULD be the next one you learn. Always push yourself... no matter what your level of ability, find the song which challenges you most and then keep on it at until you can do it in your sleep. Then move on...
The most difficult bass song in the world ever is the very next one you learn. After 34 years with either a guitar or a bass in my hands and with well over a thousand performances under my belt I am constantly finding stuff I can't yet do, things which challenge me as a musician and as a performer.
I have more I am yet to learn than I have already learned... And thank God for it... If I thought for one moment that I'd achieved all that I could as a musician I'd give it all up.