The best ever? The Intel Celeron 300a comes to mind. By changing the FSB from 66MHz to 100, you turned your cheap 300MHz CPU into a fast 450MHz CPU. At the time, Intels fastest ran at 450-500MHz, so this was a big deal. Another good Intel CPU to overclock would be the 805. For awhile, it was the only dual core CPU that existed that was under $100. People could overclock it to 4GHz, though you really needed water cooling to do that. (and even then it ran hot as h3ll.) People later realized that using the 820 was better.
AMD had their 2500XP. This one was simple, change the FSB from 166 to 200. Many 2500s could handle this, and you turned your lowest end Barton core CPU into the highest end 3200+. There were many other popular Athlons, including the 2100+ or the 1600+.
If you mean current CPUs, the Q6600 and 7200 would be current favorites. Many of these are able to hit close to 4GHz. Considering the price you pay compared to other chips, they are good deals.