WHIP has got to be #1. ERA can be too easily effected by the quality of the fielders behind a pitcher.
WHIP is not immune to that, but is closer than ERA by a large margin.
K/BB would probably be next in my book.
I'm rather vague on what the difference between ERA and ERA+ is. Could someone
elucidate it for me?
ERA+ is park and league adjusted.
Adjusted ERA+ - Wikipedia, the free encyclopedia
A 3.00 ERA in 1968 isn't quite the same as a 3.00 ERA in 1999.