None, since the registry doesn't need to be cleaned. Also, probably 95% of all registry "cleaners" don't really do anything at all besides provide a nice "feel good" report about fixing X number of errors.
If, and it's a big if, you can find one that states in very clear terms what it considers to be an "error" in the registry, and then what it does to correct these "errors" in equally clear terms... If you can find a program that states these things in no uncertain terms, you might have stumbled across a program worth having. I just wouldn't go holding your breath in anticipation of finding any such program, because the simple fact is, the registry does NOT need to be cleaned.
In general, people would be much better off if they didn't even know the registry existed, because then they wouldn't feel so compelled to go messing around with it. It's just a very simple database, I kid you not. It's nothing special or spectacular in any way. It has so little to do with system performance and stability you may as well say it has nothing to do with either.
Forget you ever heard about the registry, your life will be much simpler. The only cleaning it ever needs will be taken care of by spyware removal programs like AdAware and Spybot. After that, you're getting into obsessive-compulsive territory on cleanliness.
I am running Win Xp with a ! Gig processor. I have tow hard drives, the master is 13.5 gig and the slave is 160 gig.
Can you recommend the best brand regiostry cleaner which is afe for my pc?