Probably FP3 matrices and planes/vectors stuff esp things like linear maps of planes etc. (Or with regards to AQA, that would be FP4 - which is awesome as its an entire module on vectores/matices).
Secondary would probably be either Differential Equations (FP2/Second order since first order are rather boring in C4) - or maybe some of the more cool things you do in A level with Complex Numbers (transformations to the w plane, loci).
Most hated would be eccentricity/directex crap regarding parabolas/hyperbolas you do in FP1.