Wow, I'm the first to mention the Celeron 300a? I never even had one.
I'd vote for my Pentium III 550e.
Core 2 Duo gets a second vote - they are still decent machines 5 years in. Used to be that 5 years in the computer was better as a door weight.
Q9550. Running at 3.4 GHz slightly below stock voltage for the last 3.5 years without any complaints.
S939 X2 3800+ Manchester core. $300, got it up to 2.5ghz (25% overclock). So it was beating the ~$800 X2 4600 and sometimes the $1000 X2 4800 despite the lower cache. CPU is going strong in a friends machine to this day.
Intel Celeron G530. The ultimate CPU of the secondth/third world countries and budget setups. Cheap and fast, good enough for any game.