For Me, All the Quest were good, I would normally get the Fighters Guild Quest line for Fighters, Warriors or Soulfire (Custom class of BattleMage or Spellword.) The Mage Quest was good for my Mage or Dark Mage (A custom class) and Thieve Guild for Ninjas and lastly the Dark Brotherhood for Ninja and Shadows (My added races.) There many different quest in Oblivion for guilds and many more can be made or got so I don't rule anyone out.
If it's the questline itself it's by far the Fighters' Guild but for other things such as rewards, I'd say the Dark Brotherhood. The Thieves Guild, I found to be really boring and I have not played through the Mages Guild but if you're into magic or enchantments than Mages Guild helps a lot.