I do enjoy a foe that, when alone, is actually a valid threat to register. Something or someone that is a pretty good reflection of my abilities as a player, takes a show of skill, effort, and cunning to defeat, and that shows up at just the right frequency to make me grin rather than groan at the thought of facing another one. Darknuts from Twilight Princess and Hunter pairs from Halo: Reach are two such enemies.
Slaughtering waves of the popcorn hordes does little more than sate the ire and bloodlust generated from having to deal with them to begin with, and enemies with cheap and irritating tactics don't even warrant an afterthought in their inevitable execution. But an enemy that will face me with both honor and might will be respected. In the possible event I lose to them, I may actually laugh aloud and dignify crushing them with my full might on the next go around instead of using the absent-minded, efficiency-focused playstyle I usually adopt.