For me, personally, the hardest portions are those that have multiple iterations, but each iteration is slightly different. For example, the last aliya of Pinchas, where the mussafim for each of the days of Succos is ever-so-slightly different (not in the number of animals brought, but in prefixes and suffixes added to/subtracted from words), is challenging to me.
(Although, oddly, the nesi’im, which is similar, doesn’t give me trouble.)
The Wolf