Most fanboys will agree that Batman's greatest arch-enemy is The Joker, and, depending on what day of the week it is, I'll usually agree. However, a really close second place (appropriately enough) has always been Two-Face.
Formerly a District Attorney and pal of Batman, Harvey Dent lost his sanity and turned to crime when one side of his face was hideously disfigured. Like his bizarrely mismatched new outfit, Dent's psyche was also split down the middle, requiring the flip of a coin to determine his good or bad actions. This constant theme of duality, beyond his chilling look and M.O., is the primary reason I enjoy Two-Face so much. He seems to embody the warring sides of human nature itself, as good and bad behavior coexist yet battle for control with the capriciousness of a coin flip.
However, much farther below the lofty metaphysical conflict of good vs. evil is another battle humanity wages on a daily basis: order vs. chaos (a.k.a. "neat vs. sloppy")...which Two-Face once again embodies here on the very clever cover of Batman #313 (1979).
Published toward the tail end of a more innocent era (before Batman comics became a steady slog of psycho-operatic angst), this Garcia Lopez illustration is a ton of fun. Channeling his inner-Felix and Oscar, Harvey Dent's "Odd Couple" decor is a joy for the eye to roam. A bright yellow couch, tidy office, clean carpeting, and tea set (!) contrasted with a filthy brown couch, broken furniture, stained carpeting, a bottle of booze, and cigarettes...with Two-Face perfectly centered between the two extremes.
As for Batman himself, it's nice (though a bit jarring) to see his costume's old blue and gray color scheme, isn't it? You'll also note that he's nicely highlighted by the room's converging perspective lines...not to mention that bit of mild symbolism as he reaches out to grab Harvey's scarred side (while staying withdrawn from his unscarred side).
"Crowning" the cover design is the classic 1970's Batman logo. With its bold block lettering and 40's-style Batman face, its symmetry fits especially well with a reflective (or warped reflective) cover like this one.
Now...if only the cover scene didn't remind me so much of that apartment I shared with three guys during college (two of us were neat, and two of us were slobs).