The best birthsign in The Elder Scrolls IV: Oblivion is widely considered to be the Atronach. This birthsign gives you +150 Magicka, a 50% chance to absorb spells cast on you, but it also comes with the downside of no Magicka regeneration. It's a high-risk, high-reward choice great for players who want a strong spellcasting potential and can manage the lack of Magicka regeneration by absorbing enemy spells. Other excellent birthsigns include:

• The Thief: +10 Agility, +10 Speed, +10 Luck, great for stealthy characters.
• The Warrior: +10 Strength, +10 Endurance, ideal for melee combatants.
• The Lady: +10 Willpower, +10 Endurance, useful for spellsword builds.
• The Mage: +50 Magicka, a straightforward choice for spellcasters without drawbacks.

The best birthsign for you can also depend on your character build and playstyle—whether you favor melee, stealth, or magic. For example, the Warrior is preferred for melee builds, while the Thief is best for stealth, and the Mage and Atronach suit magic-oriented characters. In summary:

• Best overall and highest potential for magic users: Atronach
• Best for stealth builds: Thief
• Best for melee builds: Warrior
• Balanced magic boost without downsides: Mage
• Good hybrid support for spellswords: Lady

Choose the Atronach if you want the strongest magical edge, but consider the others based on your playstyle and build preferences. The choice is locked in just after the tutorial, so pick carefully. This information reflects the consensus from multiple guides and tier lists on Oblivion birthsigns.