Certain types of dubbing that resist binding together.
"Fine and dry"-style of dubbing is just fine (e.g. the kind that is wispy but still relatively dense), but other types-- like with certain types of hare's ear, for example, I feel I waste more dubbing with it because equal parts of it end of floating away or speading out across my desk then are actually caught in the dubbing loop, or adhere to the wax. Like I said before, some blends of rabbit fur are okay, but most aren't. Angora is another one that tends to float away on me.
CDC is an interesting one. I think that CDC is pretty easy to work with, but then again it just might be my luck. I feel that when you're tying with CDC- when making wings, for example- it is harder to screw up with CDC then when using something like elk hair, where you have to use a stacker and then bind it just right. On the flip, side I completely understand why some don't like working with it due to the fragility, etc.
Come to think of, I despise working with epoxy. DESPISE it. I'll use it when I have to, but I have to be in the right frame of mind to do it. It just never seems to set up the way I want it to. Consequently, most of the time I use UV Knot Sense with a thin topping layer of clear nail polish for copper johns and other similar patterns.
Deer (and elk), followed by biots. Nothing else comes close for me.
With deer and elk, it's always a struggle between securing it enough, managing flare, and managing bulk. For biots, it's always flexibility vs rib/no-rib, or in the case of tails, getting them to split nicely. I still can't tie two of them in at once, which makes it pretty time consuming.