It depends.
If we´re talking in terms of %, definetely the 1.8A-C1, running @ 3.6.
I managed to push it up to 3.345 GHz, with intel´s hsf, while a friend, with prometeia cooling, hit the 3.6. You can read the full story @ the overclockers forum.
Now if we´re talking about the top frequency, then it has to be something in the 4.4 - 4.8 Gigs range. LN2 of course, not exactly 24x7.