Mephala gives you a recipe for poison you can make with common household chemicals, tells you to put on your sixiest ninja costume, and stab your enemy to death at the stroke of midnight.
Boethiah tells you to study his daily routine, corner him when absolutely nobody can come to his aid, and violently kill him in single combat.
Molag Bal says that you should take his family hostage and force him to shoot himself. Then kill the family of course. [censored] optional (before or after). OK, not really optional.
Merhunes Dagon gives you schematics for a car bomb that would take out the entire block. Or maybe the entire town. Or maybe crack the planetary crust.
Malacath realizes that you are not an orc, turns around and leaves, muttering something about "weak mortals not able to deal with suffering".
Jyggalag also leaves, but not before sternly admonishing you for trying to spread chaos.
Hircine gives you lycanthropy, obviously. The downside is that you have to wait until the full moon, and you might eat more than just the guy down the street. Well, that, and also a mob with torches and pitchforks may show up later.
Periyite infects you with something that doesn't even have a name yet. You proceed to morosely spread His Word and His Plague to the guy down the street, his neighbors, his neighbors' neighbors... you get the idea.
Hermaeus Mora gives you a book that contains EVERYTHING about the guy you hate in excruciating detail. After reading it, you realize that 1) you own that guy's life from now on and 2) this book was so very well written that you want to read another. Hermaeus Mora gives you another book, which is even better. You never leave the house and read books for the remainder of your existence.
Vaermina offers to plague the guy with the most horrid nightmares possible until he snaps. The catch is that you'll have the exact same nightmares. Hope he snaps first.
Meridia quickly loses interest in helping you because your enemy is not undead and YOU are not a sixay Ayelid.
Namira is likewise not interested, but asks you to give her a call after the guy is actually dead.
Clavicus Vile gives you the Umbra and tells you to go to town. Unfortunately, he also casts a permanent Soul Trap on you before leaving.
Azura is very stern and imperious, though relatively polite (for a Daedra, anyway). She wants to talk, so you offer her tea and scones. She proceeds to tell you that the guy that you hate will soon discover a cure for cancer, so if you kill him - no cure for cancer. But his cure will save a child who will grow up to be a murderous maniac with body count in the hundreds. However, a victim of the murderous maniac who escapes with his life becomes a great poet who inspires a nation... you get the idea. Three days later she's still talking, you are out of tea and scones, and are completely paralyzed with indecision.
Nocturnal shows up, gets naked, stands in front of you for ten minutes, says nothing, gets dressed and disappears. She is without a doubt beautiful, but the whole thing leaves you puzzled and suspicious. Somehow you know that it's exactly what she had intended.
Sanguine enthusiastically agrees that the guy is a jerk, but revenge plans are best not made on an empty stomach. You head to the nearest bar to talk the whole thing over a beer. A week or so later, you wake up naked in a ditch in Tijuana, there's a donkey nearby that looks at you funny, and OH MY GOD TURN THE LIGHTS OFF PLEASE *bleeearrghhh* get me some water.
Sheogorath water doggy biscuits baked chicken northwest possible. Life or periwinkle blankets, whose shoe is that? Let's sunshine beetles bloody intestines sideways three. And pie. Coconuts?..