What is the best shield in oblivion

It depends on what effects you aim to have.
For example: do I want a character focused on reflect damage, reflect spell, spell absorption, max magicka... ?
In other words, there is no "best" shield, you have to choose for yourself what suits you best.

The two best shields I'd recommend are:

Spell breaker: A Daedric artifact granted for completing Peryite's quest.
It has 30% reflect spell.
Advantages: good armor rating, already obtainable at level 10.
Disadvantages: heavy armor, so weights a lot until you get the skill up.
Combine with: a grand necklace of reflection and a mundane ring to get some excellent spell reflection bonuses.

Escutcheon of Chorrol: Awarded for completing the Chorrol Sins of the Father quest.
Leveled item, so wait until level 25 if you want the strongest version.
It provides 35% reflect damage.
Advantages: good armor rating, looks nice (Ebony style),
Disadvantages: same as spell breaker plus gotta wait until level 25.
Combine with: a necklace of swords OR amulet of axes and a ring of the iron fist to become immune to all melee damage.

Hope it helps.
The Deadric shield, found in Oblivion gates. And the best sword is Umbra which you get from one of the Deadric lord's statues.
It's Irrellevant..because I finished the whole game without a shield at all.