As a raid leader with skinning, no trash mob can ever earn as much of my hatred as the core hounds in Molten Core. The Ultraxion trash was probably the most boring so far, but I can't say that I ever really hated it as I'd just watch a YouTube clip on the other monitor while it was going on.
As for good trash, I liked the trash packs before Halfus Wyrmbreaker. It requires you to actually pay attention and use CC, interrupts, dispels, etc. The early Tempest Keep trash was similar, but in TK we always had that one hunter who broke all the CC with multishot (kicking him didn't help, because you'd just end up recruiting another huntard to replace him).