I've come to realize it's really really hard to resist GPUs. For portability across different CPU architectures,
I haven't seen anything more resistant than Cuckoo Cycle. If you're willing to target recent x86 exclusively,
then you can increase resistance by employing the AES-NI instruction. Replacing the underlying siphash in Cuckoo Cycle by an AES-NI based hash function would likely render it much more GPU resistant. But I don't like the idea of excluding vast markets of potential hashing power (like smartphones while charging overnight).
Hmmm - smartphone mining would be a splendid feature . . . any idea how much more efficient a GPU vs a CPU setup would be - say based on the cost of purchase of most efficient GPU vs CPU setup + 1 year of electric fees. If the GPU setup was 2X more efficient, I could live with that for the advantage of a wide variety of mining equipment (phones, game consoles etc), but if it was 10X, not so workable.