Probably Low% TPE or All Major Endings, but I wouldn't recommend either as a beginner.

I had a blast doing Low% Genocide if your looking for a clear defined challenge without too much new in the way of actual speedrun tricks and strats. I'd recommend trying for a deathless run as a first goal.

If you really are interested in the hardest category, the answer is almost certainly True 100%. https://www.speedrun.com/undertalecategoryextensions#True_100

I would definitely suggest not trying to start out with the hardest category, though. Undertale can be an extremely technical game with all the glitches so it's generally good to start out with a shorter category you can get through easier, like Neutral.

either neutral or tpe then, since neutral is pretty optimised even though its still pushable, and tpe is neutral with some extra stuff and then true lab/asriel right. They are difficult due to being extremely technical and glitchy, requiring alot of knowledge of how the game works.