Of those I have, these are the highest CPU workouts when loaded with all samples and full releases and played on all perspectives (meaning duplicate rank samples recorded at different mic positions):
PG Nancy (4 perspectives)
PG Dusseldorf (3 perspectives)
SP San Francisco Skinner (3 perspectives with convolution reverb added to Direct ranks)
Usually, a high number of ranks (including all perspectives) and long reverb releases makes for a good test. San Francisco also has a heavy processing wind model.
SP Billerbeck Vol 1, 2 and 3 with 4 perspectives may be a good one. I only have the 3 perspective version.
SP Doesburg with 3 perspectives is another heavy load.
(I'm mentioning alternatives in case you already own one. None of these are cheap.)
You can always make a big CPU load from a large sampleset by adding more perspectives in Rank Voicing, and sending the added perspectives to several buses with long convolution reverbs added.
However, since convolution doesn't count against the polyphony limit, it's harder to uniformly report and compare results as in "this processor handles nnnnn max polyphony with CPU meter green at 96k/1024."