What is your most difficult to find Pentium processor that you have either found or are still looking for? For me bar none was the Pentium Overdrive 180MHz:
This processor must have been something of an afterthought release...something to appease the few people left at the time still running 50 and 60MHz FSB systems. When I found it existed and could upgrade my Gateway P5-120 (originally 120MHz system) I was on a mission. What a mission it was. I was extremely lucky and found ONE, never to find another again!
Other than that, most PIII Slot 1 100MHz FSB CPU's 750MHz and faster have been challenging to find!