Despite the general sway towards overkill in the majority of posts on this tread I think I prefer underkill.
It just seems so cool to be able to kill people with almost no effort in an elegant and relaxed way. That being said there always needs to be a bit of overkill if there is a perfect opportunity and it can be done stealthily, maybe 1/15 kills being massive overkill is, in my opinion, the perfect combination.