Whichever version works and gives you the highest hashes per second...
http://yacoinwiki.tk/index.php/Mining_Hardware_Comparison is the comparison page for hardware.
Even a low end GPU's gives better performance per watt than CPUs, but mine with it if ya got it and want to. YAC is highly profitable if you know what hardware excels at it and how to maximize what you've got. I keep no secrets, but I've put a lot of time and effort into my configurations and updates to the software to cope with the high N factor, and I'm going to reap those rewards now.
The Nocona seems to give me the best average around .35 Kh/s about 1/100th of what I would get doing Scrypt.
It's not too profitable but it's still decent for cpu. Better than the rest, right now.
What gpus do you have? Even low end cards mine yac decently.
The only Nvidia's I still have are a couple 8800gs's I used to fold on back in the day (get about 13khs on scrypt @ .5 kw per Kh). sold the rest. Doubt it's worth it on them although the cpu also does .5 kw per kh on scrypt so I haven't done the math for this coin as of yet.
I have been buying ATI's for the last month jumping on the alt craze. Tough to find any though!
Thanks for the info guys I'll mine cpu for a day and see what I can come up with.