Very old converts (until 2010 I'd say) are usually a bit clunky to play. They're not especially hard, but since there wasn't really well established mapping criteria, they're usually not very good converts (they're great challenge for high CS + low AR though)
I can't say it's the worst cause I haven't played all converts, but my least favorite convert is probably this one:
beatmapsets/1217044#fruits/2556534Pros:
- I like the song...
Cons:
- ...however it's kinda ruined by Nightcore. Kesha just doesn't work well with Nightcore imo
- Cut version. I'd rather play the full version of a music
- Background with a (again, opinion) terrible character
- Sliders. As a PF player, they are a pain
- Not particularly anything interesting in it. Just your average short jump map ruined by droplet placement
Only the last two points are important, but it's the combination of those four drawbacks which makes me dislike the map. Maybe it's better in Standard, but it just doesn't work as a convert.