A friend of mine brought to my attention a discussion he was having with one of our mutual friends--is pride the gravest of the seven sins? The one who told me the story believed it is, but apparently his interlocutor disagreed. Because the latter was not there to defend himself, I never did hear his side of the story, but I came into the discussion already agreeing with the former.
Pride is the gravest of the seven deadly sins for a few reasons. To begin, a definition of pride: to think that one knows better than God. This makes the first sin of the angels and the first sin of man both sins of pride, since Tradition holds that the Fall of the Angels occurred after God showed them His plan for the Incarnation and they disagreed with it. The first sin of man is clearly one of pride because Eve thought she knew better than what God told her--and then Adam followed suit, not even thinking to ask God or to correct Eve.
In addition to this fact, pride is the only sin the angels can commit because the other six are corporeal. Pride is a sin of the spirit; the others are, primarily, sins of the flesh. This makes it a more grave one because it is more insidious--a sin of the spirit is more difficult to "weed out."
One last point, which I am still looking to flesh out more fully and would appreciate some help with (if you agree, of course): pride is the root of the other six. The other six deadly sins are simply "pride-plus." Pride with an element of something else--but at their root, they are still pride. Flavours of it, if you will.
...this is kind of an ongoing thought process, so it hasn't been finished completely, but it's fleshed-out enough to post here and discuss, if anyone so chooses.