Intel® Core™ i7-3960X Processor Extreme Edition (15M Cache, up to 3.90 GHz)
is the most powerful single x86 processor available now. Intel® Xeon® Processor E5-2687W (20M Cache, 3.10 GHz, 8.00 GT/s Intel® QPI)
is the most powerful two-way x86 processor available now.
Highest clockspeed I know of belongs to IBM POWER6. It has several models clocked at 5.0 GHz but they're not faster than the above Core i7/Xeon as far as performance is concerned.
Non-x86, you'd have to look at the IBM POWER7 795. I have no idea how it stacks up to the Core i7/Xeon above but there is no denying that it is a multithreading monster
Xeons and Opterons are put through more strict quality controls than the rest. The same processors sold as consumer chips would likely have higher clockspeeds.