Which color key is the scariest/hardest?

[Slartibartfast]

For me, the intuitive order was for the blue key to be the first, red the intermediary, and yellow the last.

 

I think this is because (IIRC) that order of priority is true for MAP03 (B-R), MAP04 (B-R-Y), MAP06 (B-R-Y), and MAP09 (B-Y) of DOOM2, so it's just a thing my impressionable eight year old-brain registered as the "supposed" order even though I knew even then that some levels "did it wrong." xD