Ordinary A level maths:
In pure, I personally would say vectors and parametric integration were the worst topics. Generally the pure was fine though.
In stats, it’s hard to call. The large data set questions (for the Edexcel board) sometimes have been ridiculously hard or have just required you to recall really niche things (see the 2021 papers - both AS and A level).
In mechanics, projectile motion was probably the hardest.
Further maths (Edexcel - CP1/2, FS1, FM1 + FP1)
CP1/2 - vectors and matrix population models were the worst.
FS1 - I honestly hated the quality of tests bit the most. Otherwise it was quite a nice optional paper with some very interesting topics.
FM1 - Oblique collisions. The less said about them, the better lol
FP1 - The loci of conic sections.